Method of using carbide and/or oxycarbide containing compositions

ABSTRACT

Compositions including carbide-containing nanorods and/or oxycarbide-containing nanorods and/or carbon nanotubes bearing carbides and oxycarbides and methods of making the same are provided. Rigid porous structures including oxycarbide-containing nanorods and/or carbide containing nanorods and/or carbon nanotubes bearing carbides and oxycarbides and methods of making the same are also provided. The compositions and rigid porous structures of the invention can be used either as catalyst and/or catalyst supports in fluid phase catalytic chemical reactions. Processes for making supported catalyst for selected fluid phase catalytic reactions are also provided.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/023,618 filed Dec. 18, 2001, now U.S. Pat. No. 6,809,229, which is acontinuation-in-part of U.S. application Ser. No. 09/615,350 filed Jul.12, 2000, now abandoned, which is a continuation-in-part of U.S.application Ser. No. 09/481,184 filed Jan. 12, 2000, now U.S. Pat. No.6,514,897 which claims benefit to and is based on U.S. ProvisionalPatent Application No. 60/115,735 filed Jan. 12, 1999, whichapplications are all hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to compositions of carbide-containing andoxycarbide-containing nanorods, carbon nanotubes including carbideand/or oxycarbide compounds, rigid porous structures including thesecompositions, and methods of making and using the same. Morespecifically, the invention relates to rigid three dimensionalstructures comprising carbide and/or oxycarbide-containing nanorods orcarbon nanotubes bearing carbides and oxycarbides, having high surfaceareas and porosities, low bulk densities, substantially no microporesand increased crush strengths. The invention also relates to using thecompositions and the rigid porous structures including thesecompositions as catalysts and catalyst supports, in heterogeneouscatalytic reactions frequently encountered in petrochemical and refiningprocesses.

2. Description of the Related Art

Heterogeneous catalytic reactions are widely used in chemical processesin the petroleum, petrochemical and chemical industries. Such reactionsare commonly performed with the reactant(s) and product(s) in the fluidphase and the catalyst in the solid phase. In heterogeneous catalyticreactions, the reaction occurs at the interface between the phases,i.e., the interface between the fluid phase of the reactant(s) andproduct(s) and the solid phase of the supported catalyst. Hence, theproperties of the surface of a heterogeneous supported catalyst areimportant factors in the effective use of the catalyst. Specifically,the surface area of the active catalyst, as supported, and theaccessibility of that surface area to reactant adsorption and productdesorption are important. These factors affect the activity of thecatalyst, i.e., the rate of conversion of reactants to products. Thechemical purity of the catalyst and the catalyst support have animportant effect on the selectivity of the catalyst, i.e., the degree towhich the catalyst produces one product from among several products andthe life of the catalyst.

Generally catalytic activity is proportional to catalyst surface area.Therefore, a high specific area is desirable. However, the surface areashould be accessible to reactants and products as well as to heat flow.The chemisorption of a reactant by a catalyst surface is preceded by thediffusion of that reactant through the internal structure of thecatalyst.

Since the active catalyst compounds are often supported on the internalstructure of a support, the accessibility of the internal structure of asupport material to reactant(s), product(s) and heat flow is important.Accessibility is measured by porosity and pore size distribution.Activated carbons and charcoals used as catalyst supports may havesurface areas of about a thousand square meters per gram and porositiesof less than 1 ml/gm. However, much of this surface area and porosity,as much as 50%, and often more, is associated with micropores, i.e.,pores with pore diameters of 2 nm or less. These pores can beinaccessible because of diffusion limitations. They are easily pluggedand thereby deactivated. Thus, high porosity material where the poresare mainly in the mesopore region, i.e., greater than 2 nm or macroporeregion, i.e., greater than 50 nm, ranges are most desirable.

It is also important that self-supported catalysts and supportedcatalysts not fracture or attrit during use because such fragments maybecome entrained in the reaction stream and must then be separated fromthe reaction mixture. The cost of replacing attritted catalyst, the costof separating it from the reaction mixture and the risk of contaminatingthe product are all burdens upon the process. In slurry phase, e.g.where the solid supported catalyst is filtered from the process streamand recycled to the reaction zone, the fines may plug the filters anddisrupt the process. It is also important that a catalyst, at the veryleast, minimize its contribution to the chemical contamination ofreactant(s) and product(s). In the case of a catalyst support, this iseven more important since the support is a potential source ofcontamination both to the catalyst it supports and to the chemicalprocess. Further, some catalysts are particularly sensitive tocontamination that can either promote unwanted competing reactions,i.e., affect its selectivity, or render the catalyst ineffective, i.e.,“poison” it. For example, charcoal and commercial graphites or carbonsmade from petroleum residues usually contain trace amounts of sulfur ornitrogen as well as metals common to biological systems and may beundesirable for that reason.

Since the 1970s carbon nanofibers or nanotubes have been identified asmaterials of interest for use as catalysts and catalyst supports. Carbonnanotubes exist in a variety of forms and have been prepared through thecatalytic decomposition of various carbon-containing gases at metalsurfaces. Nanofibers such as fibrils, bucky tubes and nanotubes aredistinguishable from continuous carbon fibers commercially available asreinforcement materials. In contrast to nanofibers, which have,desirably large, but unavoidably finite aspect ratios, continuous carbonfibers have aspect ratios (L/D) of at least 10⁴ and often 10⁶ or more.The diameter of continuous fibers is also far larger than that ofnanofibers, being always greater than 1 μm and typically 5 μm to 7 μm.

U.S. Pat. No. 5,576,466 to Ledoux et al. discloses a process forisomerizing straight chain hydrocarbons having at least 7 carbon atomsusing catalysts which include molybdenum compounds whose active surfaceconsists of molybdenum carbide which is partially oxidized to form oneor more oxycarbides. Ledoux et al. disclose several ways of obtaining anoxycarbide phase on molybdenum carbide. These methods require theformation of molybdenum carbides by reacting gaseous compounds ofmolybdenum metal with charcoal at temperatures between 900° C. and 1400°C. These are energy intensive processes. Moreover, the resultingmolybdenum carbides have many drawbacks similar to catalysts preparedwith charcoal. For example, much of the surface area and porosity of thecatalysts is associated with micropores and as such these catalysts areeasily plugged and thereby deactivated.

While activated charcoals and other materials have been used ascatalysts and catalyst supports, none have heretofore had all of therequisite qualities of high surface area, porosity, pore sizedistribution, resistance to attrition and purity for the conduct of avariety of selected petrochemical and refining processes. Although manyof these materials have high surface area, much of the surface area isin the form of inaccessible micropores.

It would therefore be desirable to provide a family ofcarbide-containing and oxycarbide containing catalysts that have highlyaccessible surface area, high porosity, and attrition resistance, andwhich are substantially micropore free, highly active, highly selectiveand are capable of extended use with no significant deactivation.

Nanofiber mats, assemblages and aggregates have been previously producedto take advantage of the increased surface area per gram achieved usingextremely thin diameter fibers. These structures are typically composedof a plurality of intertwined or intermeshed nanotubes.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a compositionincluding a multiplicity of oxycarbide nanorods having predominatelydiameters between 2.0 nm and 100 nm.

It is a further object of the present invention to provide anothercomposition including a multiplicity of carbide nanorods comprisingoxycarbides.

It is a further object of the present invention to provide anothercomposition including a multiplicity of carbon nanotubes which havepredominantly diameters between 2.0 nm and 100 nm, which nanotubescomprise carbides and optionally also oxycarbides.

It is a further object of the present invention to provide anothercomposition including a multiplicity of carbon nanotubes having acarbide portion and optionally an oxycarbide portion.

It is a further object of the present invention to provide rigid porousstructures which comprise compositions including a multiplicity ofoxycarbide nanorods or a multiplicity of carbide nanorods with orwithout oxycarbides.

It is a further object of the present invention to provide compositionsof matter which comprise three-dimensional rigid porous structuresincluding oxycarbide nanorods, carbide nanorods, carbide nanorodscomprising oxycarbides, or carbon nanotubes comprising a carbide portionand optionally an oxycarbide portion.

It is a further object of the present invention to provide methods forthe preparation of and using the rigid porous structures describedabove.

It is still a further object of the invention to provide improvedcatalysts, catalyst supports and other compositions of industrial valuebased on composition including a multiplicity of carbide nanorods,oxycarbide nanorods and/or carbon nanotubes comprising carbides andoxycarbides.

It is still a further object of the invention to provide improvedcatalysts, catalyst supports and other compositions of industrial valuebased on three-dimensional rigid carbide and/or oxycarbide porousstructures of the invention.

It is an object of the invention to provide improved catalytic systems,improved catalyst supports and supported catalysts for heterogeneouscatalytic reactions for use in chemical processes in the petroleum,petrochemical and chemical industries.

It is a further object of the invention to provide improved methods forpreparing catalytic systems and supported catalysts.

It is another object of the invention to improve the economics andreliability of making and using catalytic systems and supportedcatalysts.

It is still a further object of the invention to provide improved,substantially pure, rigid carbide catalyst support of high porosity,activity, selectivity, purity and resistance to attrition.

The foregoing and other objects and advantages of the invention will beset forth in or will be apparent from the following description anddrawings.

SUMMARY OF THE INVENTION

The present invention is in compositions comprising carbide nanorodswhich contain oxycarbides. Another composition of the present inventioncomprises carbide-containing nanorods which also contain oxycarbides.Another composition comprises carbon nanotubes which bear carbidesand/or oxycarbides on the surfaces thereof. In one composition thecarbides retain the structure of the original aggregates of carbonnanotubes. Compositions are also provided which includecarbide-containing nanorods where the morphology of the aggregates ofcarbon nanotubes is not retained. The invention also provides acomposition of carbides supported on carbon nanotubes where a portion ofthe carbon nanotubes have been converted to carbide-containing nanorodsand/or carbides.

The present invention also provides rigid porous structures includingoxycarbide nanorods and/or carbide-containing nanorods and/or carbonnanotubes bearing carbides and oxycarbides. Depending on the morphologyof the carbon nanotubes used as a source of carbon, the rigid porousstructures can have a uniform or non-uniform pore distribution.Extrudates of oxycarbide nanorods and/or carbide-containing nanorodsand/or carbon nanotubes bearing oxycarbides and/or carbides are alsoprovided. The extrudates of the present invention are glued together toform a rigid porous structure.

The compositions and rigid porous structures of the invention can beused either as catalysts and/or catalyst supports in fluid phasecatalytic chemical reactions.

The present invention also provides methods of makingoxycarbide-containing nanorods, carbide-containing nanorods bearingoxycarbides and carbon-nanotubes bearing carbides and oxycarbides.Methods of making rigid porous structures are also provided. Rigidporous structures of carbide-nanorods can be formed by treating rigidporous structures of carbon nanotubes with a Q-containing compound,i.e., a compound containing a transition metal, a rare earth or anactinide. Depending upon conditions the conversion of the carbonnanotubes to carbide-containing nanorods can be complete or partial. Therigid porous structure of carbide nanorods and/or carbon nanotubes canbe further treated with an oxidizing agent to form oxycarbide nanorodsand/or oxycarbides. The rigid porous structures of the invention canalso be prepared from loose nanorods or aggregates of carbide-containingnanorods and/or oxycarbide-containing nanorods by initially forming asuspension in a medium, separating the suspension from the medium, andpyrolyzing the suspension to form rigid porous structures. The presentinvention also provides a process for making supported catalysts forselected fluid phase reactions.

Other improvements which the present invention provides over the priorart will be identified as a result of the following description whichsets forth the preferred embodiments of the present invention. Thedescription is not in any way intended to limit the scope of the presentinvention, but rather only to provide a working example of the presentpreferred embodiments. The scope of the present invention will bepointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an exemplary embodiment of thepresent invention.

FIG. 1A is an XRD graph of sample 12 as set forth in Table 1. Areference XRD pattern of hexagonal Mo₂C is shown immediately below.

FIGS. 1B and 1C are SEM micrographs of sample 12 as set forth in Table1.

FIG. 2A is an XRD graph of sample 12 as set forth in Table 1. Areference XRD pattern of hexagonal Mo₂C is also shown immediately below.

FIG. 2B is an HRTEM micrograph of sample 12 as set forth in Table 1.

FIG. 3A is an XRD graph of sample 10 as set forth in Table 1. ReferenceXRD patterns of hexagonal Mo₂C, cubic Mo₂C and graphite are shownimmediately below.

FIG. 3B is an HRTEM micrograph of sample 10 as shown in Table C.

FIG. 4 is a thermogravimetric analysis of sample 12 as set forth inTable 1.

FIG. 5A is an SEM micrograph of SiC extrudates.

FIG. 5B is an SEM micrograph illustrating micropores among theaggregates of the extrudates shown in FIG. 5A.

FIG. 5C is an SEM micrograph illustrating micropores in the networks ofthe intertwined SiC nanorods present in the extrudates shown in FIG. 5A.

DETAILED DESCRIPTION OF THE INVENTION

Patents, patent applications, and patent publications are referred toherein are incorporated by reference in their entirety.

Definitions

“Aggregate” refers to a dense, microscopic particulate structure. Morespecifically, the term “assemblage” refers to structures havingrelatively or substantially uniform physical properties along at leastone dimensional axis and desirably having relatively or substantiallyuniform physical properties in one or more planes within the assemblage,i.e. they have isotropic physical properties in that plane. Theassemblage may comprise uniformly dispersed individual interconnectednanotubes or a mass of connected aggregates of nanotubes. In otherembodiments, the entire assemblage is relatively or substantiallyisotropic with respect to one or more of its physical properties. Thephysical properties which can be easily measured and by which uniformityor isotropy are determined include resistivity and optical density.

“Bimodal pore structure” refers to a specific pore structure occurringwhen aggregate particles of nanotubes and/or nanorods are bondedtogether. The resulting structure has a two-tiered architecturecomprising a macrostructure of nanotube aggregates having macroporesamong the bundles of nanotube aggregates and a microstructure ofintertwined nanotubes having a pore structure within each individualbundle of aggregate particles.

“Carbides” refers to compounds of composition QC or Q₂C. The term alsoincludes crystalline structures characterized by x-ray diffraction(“XRD”) as QC or Q₂C by themselves and/or in combinations with Q or C,e.g., compounds remaining after the synthesis step is substantiallycomplete. Carbides can be detected and characterized by XRD. When, as iscontemplated within the scope of this invention, the carbides areprepared by carburization of metal oxides or by oxidation of elementalcarbon, a certain amount of “non-stoichiometric” carbide may appear, butthe diffraction pattern of the true carbides will still be present.Metal rich non-stoichiometric carbides, such as might be formed from asynthesis wherein the metal is carburized, are missing a few of thecarbons that the metal matrix can accommodate. Carbon richnon-stoichiometric carbides comprise domains of stoichiometric carbidesembedded in the original carbon structure. Once the carbide crystallitesare large enough they can be detected by XRD.

Carbides also refers to interstitial carbides as more specificallydefined in Structural Inorganic Chemistry, by A. F. Wells, 4th Ed.,Clarendon Press, Oxford 1975 and in The Chemistry of Transition MetalCarbides and Nitrides, edited by S. T. Oyama, 1st Ed., a BlackieAcademic & Professional publication 1996. Both books are herebyincorporated by reference in their entirety.

“Carbide-containing nanorod” refers to a Q-containing nanorodpredominantly having a diameter greater than 2 nm but less than 100 nm,for example greater than 5 nm but less than 50 nm, and having an aspectratio from 5 to 500. When the carbide nanorod has been made byconversion of the carbon of a nanotube to carbide compounds then theconversion has been substantially complete.

“Fluid phase reaction” refers to any liquid or gas phase catalyticreactions such as hydrogenation, protonation, oxidation,hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation,hydrodeoxygenation, protonation, hydrodearomatization, dehydrogenation,hydrogenolysis, isomerization, alkylation, dealkylation, andtransalkylation.

“Graphenic” carbon is a form of carbon whose carbon atoms are eachlinked to three other carbon atoms in an essentially planar layerforming hexagonal fused rings. The layers are platelets having only afew rings in their diameter or ribbons having many rings in their lengthbut only a few rings in their width.

“Graphenic analogue” refers to a structure which is incorporated in agraphenic surface.

“Graphitic” carbon consists of layers which are essentially parallel toone another and no more than 3.6 angstroms apart.

“Internal structure” refers to the internal structure of an assemblageincluding the relative orientation of the fibers, the diversity of andoverall average of nanotube orientations, the proximity of the nanotubesto one another, the void space or pores created by the interstices andspaces between the fibers and size, shape, number and orientation of theflow channels or paths formed by the connection of the void spacesand/or pores. According to another embodiment, the structure may alsoinclude characteristics relating to the size, spacing and orientation ofaggregate particles that form the assemblage. The term “relativeorientation” refers to the orientation of an individual nanotube oraggregate with respect to the others (i.e., aligned versus non-aligned).The “diversity of” and “overall average” of nanotube or aggregateorientations refers to the range of nanotube orientations within thestructure (alignment and orientation with respect to the externalsurface of the structure).

“Isotropic” means that all measurements of a physical property within aplane or volume of the structure, independent of the direction of themeasurement, are of a constant value. It is understood that measurementsof such non-solid compositions must be taken on a representative sampleof the structure so that the average value of the void spaces is takeninto account.

“Macropore” refers to a pore which has a diameter of greater than orequal to 50 nm.

“Mesopore” refers to a pore which has a diameter of greater than orequal to 2 nm but less than 50 nm.

“Micropore” refers to a pore which has a diameter of less than 2 nm.

“Nanorod” refers to a rod-like structure having a surface and asubstantially solid core with a diameter of at least 1 nm but less than100 nm. The structure has an aspect ratio between 10 and 500 and alength up to 50 μm. The diameter of a nanorod is substantially uniformalong the entire length of the nanorod. A nanorod is solid not hollow.

“Nanostructure” refers to nanotubes, nanorods, and any combinations ormixtures of nanotubes and nanorods.

“Nanotube”, “nanofiber” and “fibril” are used interchangeably. Eachrefers to an elongated hollow structure having a diameter less than 1μm. The term “nanotube” also includes “bucky tubes” and graphiticnanofibers in which the graphene planes are oriented in herring bonepattern.

“Nonuniform pore structure” refers to a pore structure occurring whenindividual discrete nanotubes are distributed in a substantiallynonuniform manner with substantially nonuniform spacings betweennanotubes.

Oxycarbides, unlike carbides, are inherently non-stoichiometric. Theyare any structure containing oxygen predominantly on the surface andderived from a carbide. For example, the oxycarbides of the presentinvention can have the formula:Q_(n)C_(x-y)O_(y)wherein Q is as defined above; n and x are selected to satisfy a knownstoichiometry of a carbide of Q; y is less than x and the ratio[y/(x-y)] is at least 0.02 and less than 0.9 and more preferably isbetween 0.05 and 0.50. Furthermore, the term “oxycarbides” alsoincludes, but is not limited to, products formed by oxidative treatmentsof carbides present in carbon nanotubes used as a source of carbon or inconnection with carbide nanorods as a source of carbides. Oxycarbidescan also include products formed by carburization of metal oxides.Oxycarbides also comprise mixtures of unreacted carbides and oxides,chemisorbed and physisorbed oxygen. More specifically, oxycarbides havea total amount of oxygen sufficient to provide at least 25% of at leastone monolayer of absorbed oxygen as determined by temperature programmeddesorption (TPD) containing on the carbide content of the carbidesource. Oxycarbides also refer to compounds of the same name as definedin The Chemistry of Transition Metal Carbides and Nitrides, edited by S.T. Oyama, a Blackie Academic & Professional Publication. Examples ofoxycarbides include polycrystalline compounds, wherein Q is a metalpreferably in two valent states. Q can be bonded to another metal atomor only to an oxygen or only to a carbon atom. However, Q is not bondedto both oxygen and carbon atoms. The term “carbides” encompasses bothcarbides and oxycarbides.

“Oxycarbides-containing nanorod” refers to an Q-containing nanorodhaving the formula Q_(n)C_(x-y)O_(y) as defined above, having an aspectratio of 5 to 500.

“Physical property” means an inherent, measurable property of the porousstructure, e.g., surface area, resistivity, fluid flow characteristics,density, porosity, etc.

“Pore” traditionally refers to an opening or depression in the surfaceof a catalyst or catalyst support. Catalysts and catalyst supportscomprising carbon nanotubes lack such traditional pores. Rather, inthese materials, the spaces between individual nanotubes behave aspores, and the equivalent pore size of nanotube aggregates can bemeasured by conventional methods (porosimetry) of measuring pore sizeand pore size distribution. By varying the density and structure ofaggregates, the equivalent pore size and pore size distribution can bevaried.

“Q” represents an element selected from the group consisting oftransition metals (groups IIIB, IVB, VB, VIB, VIIB, and VIII of periods4, 5, and 6 of the Periodic Table), rare earths (lanthanides) andactinides. Q can also be boron, silicon or aluminum More preferably, Qis selected from the group consisting of Ti, Ta, Nb, Zr, Hf, Mo, V, B,Si, Al and W.

“Q-containing” refers to a compound or composition modified reactionwith Q as defined above.

“Relatively” means that 95% of the values of the physical property whenmeasured along an axis of, or within a plane of or within a volume ofthe structure, as the case may be, will be within plus or minus 20% of amean value.

“Substantially” or “predominantly” mean that 95% of the values of thephysical property when measured along an axis of, or within a plane ofor within a volume of the structure, as the case may be, will be withinplus or minus 10% of a mean value.

“Surface area” refers to the total surface area of a substancemeasurable by the BET technique as known in the art, a physisorptiontechnique. Nitrogen or helium can be use absorbents to measure thesurface area.

“Uniform pore structure” refers to a pore structure occurring whenindividual discrete nanotubes or nanofibers form the structure. In thesecases, the distribution of individual nanotubes in the particles issubstantially uniform with substantially regular spacings between thenanotubes. These spacings (analogous to pores in conventional supports)vary according to the densities of the structures.

Carbon Nanotubes

The term nanotubes refers to various carbon tubes or fibers having verysmall diameters including fibrils, whiskers, buckytubes, etc. Suchstructures provide significant surface area when assembled into astructure because of their size and shape. Moreover, such nanotubes canbe made with high purity and uniformity.

Preferably, the nanotube used in the present invention have a diameterless than 1 μm, preferably less than about 0.5 μm, and even morepreferably less than 0.1 μm and most preferably less than 0.05 μm.

Carbon nanotubes can be made having diameters in the range of 3.5 to 70nm.

The nanotubes, buckytubes, fibrils and whiskers that are referred to inthis application are distinguishable from continuous carbon fiberscommercially available as reinforcement materials. In contrast tonanofibers, which have desirably large, but unavoidably finite aspectratios, continuous carbon fibers have aspect ratios (L/D) of at least10⁴ and often 10⁶ or more. The diameter of continuous fibers is also farlarger than that of fibrils, being always greater than 1 μm andtypically 5 to 7 μm.

Continuous carbon fibers are made by the pyrolysis of organic precursorfibers, usually rayon, polyacrylonitrile (“PAN”) and pitch. Thus, theymay include heteroatoms within their structure. The graphitic nature of“as made” continuous carbon fibers varies, but they may be subjected toa subsequent graphitization step. Differences in degree ofgraphitization, orientation and crystallinity of graphite planes, ifthey are present, the potential presence of heteroatoms and even theabsolute difference in substrate diameter make experience withcontinuous fibers poor predictors of nanofiber chemistry.

Carbon nanotubes exist in a variety of forms and have been preparedthrough the catalytic decomposition of various carbon-containing gasesat metal surfaces.

U.S. Pat. No. 4,663,230 to Tennent hereby incorporated by reference,describes carbon nanotubes that are free of a continuous thermal carbonovercoat and have multiple ordered graphitic outer layers that aresubstantially parallel to the nanotube axis. As such they may becharacterized as having their c-axes, the axes which are perpendicularto the tangents of the curved layers of graphite, substantiallyperpendicular to their cylindrical axes. They generally have diametersno greater than 0.1 μm and length to diameter ratios of at least 5.Desirably they are substantially free of a continuous thermal carbonovercoat, i.e., pyrolytically deposited carbon resulting from thermalcracking of the gas feed used to prepare them. Tennent describesnanotubes typically 3.5 to 70 nm having an ordered, “as grown” graphiticsurface.

U.S. Pat. No. 5,171,560 to Tennent et al., hereby incorporated byreference, describes carbon nanotubes free of thermal overcoat andhaving graphitic layers substantially parallel to the nanotube axes suchthat the projection of the layers on the nanotube axes extends for adistance of at least two nanotube diameters. Typically, such nanotubesare substantially cylindrical, graphitic nanotubes of substantiallyconstant diameter and comprise cylindrical graphitic sheets whose c-axesare substantially perpendicular to their cylindrical axis. They aresubstantially free of pyrolytically deposited carbon, have a diameterless than 0.1 μm and a length to diameter ratio of greater than 5. Thesefibrils are of primary interest in the invention.

When the projection of the graphitic layers on the nanotube axis extendsfor a distance of less than two nanotube diameters, the carbon planes ofthe graphitic nanotube, in cross section, take on a herring boneappearance. These are termed fishbone fibrils. U.S. Pat. No. 4,855,091to Geus, hereby incorporated by reference, provides a procedure forpreparation of fishbone fibrils substantially free of a pyrolyticovercoat. These carbon nanotubes are also useful in the practice of theinvention. See also, U.S. Pat. No. 5,165,909 to Tennent, herebyincorporated by reference.

Oxidized nanofibers are used to form rigid porous assemblages. U.S. Pat.No. 5,965,470, hereby incorporated by reference, describes processes foroxidizing the surface of carbon nanotubes that include contacting thenanotubes with an oxidizing agent that includes sulfuric acid (H₂SO₄)and potassium chlorate (KClO₃) under reaction conditions (e.g., time,temperature, and pressure) sufficient to oxidize the surface of thefibril. The nanotubes oxidized according to the processes of McCarthy,et al. are non-uniformly oxidized, that is, the carbon atoms aresubstituted with a mixture of carboxyl, aldehyde, ketone, phenolic andother carbonyl groups.

Nanotubes have also been oxidized nonuniformly by treatment with nitricacid. International Application WO95/07316 discloses the formation ofoxidized fibrils containing a mixture of functional groups. Hoogenvaad,M. S., et al. (Metal Catalysts Supported on a Novel Carbon Support,Presented at Sixth International Conference on Scientific Basis for thePreparation of Heterogeneous Catalysts, Brussels, Belgium, September1994) also found it beneficial in the preparation of nanotube-supportedprecious metals to first oxidize the nanotube surface with nitric acid.Such pretreatment with acid is a standard step in the preparation ofcarbon-supported noble metal catalysts, where, given the usual sourcesof such carbon, it serves as much to clean the surface of undesirablematerials as to functionalize it.

In published work, McCarthy and Bening (Polymer Preprints ACS Div. ofPolymer Chem. 30 (1)420(1990)) prepared derivatives of oxidizednanotubes in order to demonstrate that the surface comprised a varietyof oxidized groups. The compounds they prepared, phenylhydrazones,haloaromaticesters, thallous salts, etc., were selected because of theiranalytical utility, being, for example, brightly colored, or exhibitingsome other strong and easily identified and differentiated signal.

Nanotubes may be oxidized using hydrogen peroxide, chlorate, nitric acidand other suitable reagents. See, for example, U.S. patent applicationSer. No. 09/861,370 filed May 18, 2001 entitled “Modification of CarbonNanotubes by Oxidation with Peroxygen Compounds” and U.S. patentapplication Ser. No. 09/358,745, filed Jul. 21, 1999, entitled “Methodsof Oxidizing Multiwalled Carbon Nanotubes.”

The nanotubes within the structure may be further functionalized asdescribed in U.S. Pat. No. 6,203,814 to Fischer.

Carbon nanotubes of a morphology similar to the catalytically grownfibrils or nanotubes described above have been grown in a hightemperature carbon arc (Iijima, Nature 354 56 1991, hereby incorporatedby reference). It is now generally accepted (Weaver, Science 265 1994,hereby incorporated by reference) that these arc-grown nanofibers havethe same morphology as the earlier catalytically grown fibrils ofTennent. Arc grown carbon nanofibers are also useful in the invention.

Nanotube Aggregates and Assemblages

The “unbonded” precursor nanotubes may be in the form of discretenanotubes, aggregates of nanotubes or both.

As with all nanoparticles, nanotubes aggregate in several stages ordegrees. Catalytically grown nanotubes produced according to U.S. Pat.No. 6,031,711 are formed in aggregates substantially all of which willpass through a 700 μm sieve. About 50% by weight of the aggregates passthrough a 300 μm sieve. The size of as-made aggregates can, of course,be reduced by various means.

These aggregates have various morphologies (as determined by scanningelectron microscopy) in which they are randomly entangled with eachother to form entangled balls of nanotubes resembling bird nests (“BN”);or as aggregates consisting of bundles of straight to slightly bent orkinked carbon nanotubes having substantially the same relativeorientation, and having the appearance of combed yarn (“CY”)—e.g., thelongitudinal axis of each nanotube (despite individual bends or kinks)extends in the same direction as that of the surrounding nanotubes inthe bundles; or, as, aggregates consisting of straight to slightly bentor kinked nanotubes which are loosely entangled with each other to forman “open net” (“ON”) structure. In open net structures the extent ofnanotube entanglement is greater than observed in the combed yarnaggregates (in which the individual nanotubes have substantially thesame relative orientation) but less than that of bird nest. CY and ONaggregates are more readily dispersed than BN.

When carbon nanotubes are used, the aggregates, when present, aregenerally of the bird's nest, combed yarn or open net morphologies. Themore “entangled” the aggregates are, the more processing will berequired to achieve a suitable composition if a high porosity isdesired. This means that the selection of combed yarn or open netaggregates is most preferable for the majority of applications. However,bird's nest aggregates will generally suffice.

The morphology of the aggregate is controlled by the choice of catalystsupport. Spherical supports grow nanotubes in all directions leading tothe formation of bird nest aggregates. Combed yarn and open nestaggregates are prepared using supports having one or more readilycleavable planar surfaces. U.S. Pat. No. 6,143,689 hereby incorporatedby reference, describes nanotubes prepared as aggregates having variousmorphologies.

Further details regarding the formation of carbon nanotube or nanofiberaggregates may be found in the disclosures of U.S. Pat. No. 5,165,909;U.S. Pat. No. 5,456,897; U.S. Pat. No. 5,707,916; U.S. Pat. No.5,877,110; PCT Application No. US89/00322, filed Jan. 28, 1989 (“CarbonFibrils”) WO 89/07163, and Moy et al., U.S. Pat. No. 5,110,693, U.S.patent application Ser. No. 447,501 filed May 23, 1995; U.S. patentapplication Ser. No. 456,659 filed Jun. 2, 1995; PCT Application No.US90/05498, filed Sep. 27, 1990 (“Fibril Aggregates and Method of MakingSame”) WO 91/05089, and U.S. Pat. No. 5,500,200; U.S. application Ser.No. 08/329,774 by Bening et al., filed Oct. 27, 1984; and U.S. Pat. No.5,569,635, all of which are assigned to the same assignee as theinvention here and of which are hereby incorporated by reference.

Nanotube mats or assemblages have been prepared by dispersing nanofibersin aqueous or organic media and then filtering the nanofibers to form amat or assemblage. The mats have also been prepared by forming a gel orpaste of nanotubes in a fluid, e.g. an organic solvent such as propaneand then heating the gel or paste to a temperature above the criticaltemperature of the medium, removing the supercritical fluid and finallyremoving the resultant porous mat or plug from the vessel in which theprocess has been carried out. See, U.S. Pat. No. 5,691,054.

Extrudates of Carbon Nanotubes

In a preferred embodiment the carbon rigid porous structures compriseextrudates of carbon nanotubes. Aggregates of carbon nanotubes treatedwith a gluing agent or binder are extruded by conventional extrusionmethods into extrudates which are pyrolyzed or carbonized to form rigidcarbon structures. If the bundles of carbon nanotubes are substantiallyintact except that they have been splayed (e.g. by sonication) orpartially unraveled, the structure provides a bimodal pore structure.The space between bundles ranges from points of contact to about 1 μm.Within bundles, spaces between carbon nanotubes range from 10 to 30 nm.The resulting rigid bimodal porous structure is substantially free ofmicropores, has surface areas ranging from about 250 m²/gm to about 400m²/gm and a crush strength of about 20 psi for extrudates of ⅛ inch indiameter. Carbon nanotube extrudates have densities ranging from about0.5 gm/cm³ to about 0.7 gm/cm³, which can be controlled by the densityof the extrusion paste. The extrudates have liquid absorption volumesfrom about 0.7 cm³/gm.

Gluing or binding agents are used to form the paste of carbon nanotubesrequired for extrusion processes. Useful gluing or binding agentsinclude, without limitation, cellulose, carbohydrates, polyethylene,polystyrene, nylon, polyurethane, polyester, polyamides,poly(dimethylsiloxane), phenolic resins and the like.

The extrudates obtained as described above can be further treated withmild oxidizing agents such as hydrogen peroxide without affecting theintegrity of the rigid porous carbon structures. Subsequently, the rigidporous structures can be impregnated with catalytic particles by ionexchange, generally a preferred method for deposition of small sizeparticles. Alternatively, the rigid porous carbon structure can also beimpregnated with catalysts by incipient wetness, or physical or chemicaladsorption.

The rigid, high porosity structures can be formed from regular nanotubesor nanotube aggregates, either with or without surface nanofibers (i.e.,surface oxidized nanotubes). Surface oxidized nanotubes can becross-linked according to methods described in U.S. Pat. No. 6,031,711and U.S. Pat. No. 6,099,965, and then carbonized to from a rigid porouscarbon structure having a uniform pore structure, substantially free ofmicropores.

Nanorods

The term nanorods refers to rod-like structures having a substantiallysolid core, a surface and a diameter greater than 1 nanometer but lessthan 100 nm. The structure has an aspect ratio between 5 and 500 and alength between 2 nm and 50 μm and preferably between 100 nm and 20 μm.The disclosed nanorods are substantially solid. They are not hollow withone open end, hollow with two open ends or hollow with two sealed ends.

Carbide Nanorods

Carbide-containing nanorods can be prepared by using carbon nanotubes asa source of carbon. For example, in WO/00/19121 incorporated herein byreference, carbide nanorods were prepared. Q-containing gas was reactedwith carbon nanotubes to form, in situ, solid Q-containing carbidenanorods at temperatures substantially less than 1700° C. and preferablyin the range of about 1000° C. to about 1400° C., and more preferably atapproximately 1200° C. The Q-containing gases were volatile compoundscapable of forming carbides.

This conversion is called pseudotopotactic because even though thedimensions and crystalline orientations of the starting material andproduct differ, the cylindrical geometry of the starting nanotube isretained in the final nanorod and the nanorods remain separate andpredominately unfused to each other. The diameters of the resultingnanorods were about double that of the starting carbon nanotubes (1nm–100 nm).

Carbide nanorods have also been prepared by reacting carbon nanotubeswith volatile metal or non-metal oxide species at temperatures between500° C. and 2500° C. wherein the carbon nanotube is believed to act as atemplate, spatially confining the reaction to the nanotube in accordancewith methods described in PCT/US 96/09675 by C. M. Lieber. See also U.S.patent application Ser. Nos. 09/615,350 and 09/481,184 filed Jul. 12,2000 and Jan. 12, 2000 respectively. Carbide nanorods formed by methodswherein the carbon nanotube serves as a template are also useful in thepresent invention.

Because of the ease with which they can penetrate fibril aggregates andrigid porous structures, volatile Q compounds are usually preferred.Volatile Q precursors are compounds having a vapor pressure of at leasttwenty torr at reaction temperature. Reaction with the volatile Qcompound may or may not take place through a non-volatile intermediate.

Other methods of preparing carbide nanorods include reductivecarburization in which the carbon nanotubes are reacted withQ-containing volatile metal oxides followed by passing a flow of gaseousCH₄/H₂ mixture at temperatures between 250° C. and 700° C. In additionto Q-containing metal oxides, volatile Q-containing compounds useful inpreparation of Q-containing carbide nanorods include carbonyls andchlorides such as, for example, Mo(CO)₆, Mo(V) chloride or W(VI)Ochloride.

In a preferred method of making useful carbide nanorods for the presentinvention, vapors of a volatile Q-containing compound are passed over abed of extrudates of carbon nanotubes in a quartz tube at temperaturesfrom about 700° C. to about 1000° C. By controlling the concentration ofthe Q-containing compound, the crystallization of the carbides islimited to the space of the nanotubes.

In all the methods of providing carbide-containing nanorods discussedabove, the extent of conversion of the carbon in carbon nanotubes tocarbide nanorods can be controlled by adjusting the concentration of theQ-containing compound, the temperature at which the reaction occurs andthe duration of the exposure of carbon nanotubes to the volatileQ-containing compound. The extent of conversion of the carbon from thecarbon nanotubes is between 0.5% and 100%, and preferably around 95%.The resulting carbide nanorods have an excellent purity level in thecarbide content, vastly increased surface area and improved mechanicalstrength. The surface area of the carbide nanorods is from 1 to 400 andpreferably 10 to 300 m²/gm.

Applications for compositions based on carbide nanorods includecatalysts and catalyst supports. For example, compositions includingcarbide nanorods based on molybdenum carbide, tungsten carbide, vanadiumcarbide, tantalum carbide and niobium carbide are useful as catalysts influid phase reactions.

Similarly, silicon carbide and aluminum carbide-containing nanorods areespecially useful as catalyst supports for conventional catalysts suchas platinum and palladium, as well as for other Q-containing carbidessuch as molybdenum carbide, tungsten carbide, vanadium carbide and thelike.

Oxycarbide Nanorods

Oxycarbide-containing nanorods can be prepared from carbide nanorods.The carbide nanorods are subjected to oxidative treatments known in theart. For example, oxidative treatments are disclosed in U.S. Pat. No.5,576,466; M. Ledoux, et al. European Pat. Appln. No. 0396 475 A1, 1989;C. Pham-Huu, et al., Ind. Eng. Chem. Res. 34, 1107–1113, 1995; E.Iglesia, et al., Journal of Catalysis, 131, 523–544, 1991. The foregoingoxidative treatments are applicable to the formation of oxycarbidenanorods as well as to the formation of nanotubes and/or nanorodscomprising an oxycarbide portion wherein the conversion of the carbidesource is incomplete.

Oxycarbide compounds present in an oxycarbide nanorod, and also presentwhen the conversion of the carbide source is incomplete, includeoxycarbides having a total amount of oxygen sufficient to provide atleast 25% of at least 1 monolayer of absorbed oxygen as determined bytemperature programmed desorption (TPD) based on the carbide content ofthe carbide source. For example, by subjecting carbide nanorods to acurrent of oxidizing gas at temperatures of between 30° C. to 500° C.oxycarbide nanorods are produced. Useful oxidizing gases include but arenot limited to air, oxygen, carbon dioxide, nitrous oxide, water vaporand mixtures thereof. These gases may be pure or diluted with nitrogenand/or argon.

Compositions comprising oxycarbide nanorods are useful as catalysts inmany fluid phase petrochemical and refining processes includinghydrogenation, protonation, oxidation, hydrodesulfurisation,hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation,hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization,alkylation, dealkylation and transalkylation.

Supported Carbides and Oxycarbides

According to another embodiment of the present invention, by adjustingthe process parameters, for example, the temperature, the concentrationof, and the length of exposure to the Q-containing volatile compound, itis possible to limit the rate of conversion of the carbon in the carbonnanotube. Thus, it is possible to provide carbon nanotubes having acarbide portion where the location of the carbide portion can beengineered as desired. For example, the carbide portion of the carbonnanotube can be located entirely on the surface of the carbon nanotubesuch that only parts of the surface comprise nanocarbide compounds. Itis possible to have the entire surface of the carbon nanotube coatedwith carbides while the core of the carbon nanotube remainssubstantially carbon. Moreover, it is possible to control the surfacecoverage of carbon nanotubes with carbide compounds at from 1% to 99% ofthe entire surface area. An embodiment wherein the carbon nanotubecomprises carbide covering less than 50% of the surface of the carbonnanotube is preferred. Of course, at low percentages large areas of thecarbon nanotube surface remain uncovered. Nevertheless, as long as thecarbide portion of the carbon nanotube is retained at the surface, themorphology of the carbon nanotube remains substantially the same.Similarly, through careful control of the process parameters, it ispossible to turn the carbide portion of the nanotube into a carbidenanorod thereby obtaining a nanotube-nanorod hybrid structure. Thecarbide portion can be located anywhere on the carbon nanotube. Partialconversion of carbon to carbide compounds preferably varies from about20% to about 85% by weight. When the content of carbide compounds in thecarbon nanotube exceeds 85% by weight, the carbon nanotubes have beensubstantially converted to carbide nanorods. Once in possession of theteachings herein, one of ordinary skill in the art can determine as aroutine matter and without the need for undue experimentation how tocontrol the rate of conversion of carbon nanotubes to carbide nanorodsin order to convert the carbon in the carbon nanotubes incompletely.

The embodiment of the invention where the carbon nanotubes contain acarbide portion also encompasses providing the carbide portion of thecarbon nanotube in any manner now known or later developed. For example,in another method of providing carbide compounds on carbon nanotubes oraggregates thereof, the Q-containing metal or metal compound, preferablymolybdenum, tungsten, or vanadium is placed on the carbon nanotubes oraggregates directly and then pyrolyzed, leaving behind carbon nanotubescoated with carbide compounds.

In yet another method of providing carbide compounds on carbonnanotubes, solutions of Q-containing salts, such as, for example, saltsof molybdenum, tungsten, or vanadium are dispersed over the carbonnanotubes or aggregates thereof and then pyrolyzed, again formingcarbide compounds primarily on the surface of the carbon nanotubes.

An embodiment wherein the core of the carbon nanotube remains carbon andthe location of the metallic carbides is limited is quite desirable as acatalytic system. The core of the carbon nanotube acts as a catalystsupport or carrier for the metallic carbide catalyst.

In yet another embodiment of the invention, it is possible to transformthe core of the carbon nanotubes into one metal carbide preferablysilicon carbide or aluminum carbide at temperatures between 1100° C. and1400° C. Thereafter, by bringing the silicon carbide nanorod in contactwith the volatile compound of another metal, for example, MoO, a mixedcarbide nanorod is provided which has a silicon carbide (preferably βSiC), core and another Q-based carbide portion. When MoO is used forexample, the SiC nanorod can have a MoC portion that could be an outerlayer or a MoC-based nanorod. Thus, the resulting nanorod is a mixedcarbide-based nanorod wherein part of the nanorod is SiC-based andanother portion is MoC-based. There is likewise an advantageous presenceof molybdenum silicide. The mixed carbide nanotube or nanorods asdiscussed above are particularly suitable as catalyst carriers ordirectly as catalysts in high temperature chemical reactions,particularly in the petrochemical field.

In yet another embodiment of the improvement discussed above, it ispossible to subject the nanotube having a carbide portion to oxidativetreatments such that the carbide portion of the nanotube furthercomprises an oxycarbide portion. The oxycarbide portion comprisesoxycarbide compounds located any place on, in and within the carbonnanotube or carbide nanorod.

The oxycarbide compounds can be placed on the nanotube in any way nowknown or later developed. Similarly, the nanotube having a carbideportion can be exposed to air or subjected to carburization or any othermeans of converting the carbide portion of the nanotube partially orcompletely into an oxycarbide nanorod portion. Thus, it is possible toprovide a carbon nanotube which is partly still a carbon nanotube,partly a carbide nanorod and partly an oxycarbide nanorod. This may bereferred to as a carbon-carbide-oxycarbide nanotube-nanorod hybrid.

Carbide and Oxycarbide Rigid Porous Structures

The invention also relates to rigid porous structures made from carbidenanorods, oxycarbide nanorods, and supported carbide and oxycarbidecarbon nanotubes and methods for producing the same. The resultingstructures may be used in catalysis, chromatography, filtration systems,electrodes, batteries and the like.

The rigid porous structures according to the invention have highaccessible surface area. That is, the structures have a high surfacearea which is substantially free of micropores. The invention relates toincreasing the mechanical integrity and/or rigidity of porous structurescomprising intertwined carbon nanotubes and/or carbide and/or oxycarbidenanorods. The structures made according to the invention have highercrush strengths than the conventional carbon nanotube or nanorodstructures. The present invention provides a method of improving therigidity of the carbon structures by causing the nanotubes and/ornanorods to form bonds or become glued with other nanotubes and/ornanorods at the nanotube and/or nanorod intersections. The bonding canbe induced by chemical modification of the surface of the nanotubes topromote bonding, by adding “gluing” agents and/or by pyrolyzing thenanotubes to cause fusion or bonding at the interconnect points.

The nanotubes or nanorods can be in the form of discrete nanotubesand/or nanorods or aggregate particles of nanotubes and nanorods. Theformer results in a structure having fairly uniform properties. Thelatter results in a structure having two-tiered architecture comprisingan overall macrostructure comprising aggregate particles of nanotubesand/or nanorods bonded together and a microstructure of intertwinednanotubes and/or nanorods within the individual aggregate particles.

According to one embodiment, individual discrete nanotubes and/ornanorods form the structure. In these cases, the distribution ofindividual nanotube and/or nanorod strands in the particles aresubstantially uniform with substantially regular spacing betweenstrands. These spacings (analogous to pores in conventional supports)vary according to the densities of the structures and range roughly from15 nm in the densest to an average 50 to 60 nm in the lightest particles(e.g., solid mass formed from open net aggregates). Absent are cavitiesor spaces that would correspond to micropores in conventional carbonsupports.

According to another embodiment, the distribution of individualnanotubes and/or nanorods is substantially nonuniform with asubstantially nonuniform pore structure. Nevertheless, there are nocavities or spaces corresponding to micropores which are frequentlypresent in other catalysts and catalyst supports.

These rigid porous materials are superior to currently available highsurface area materials for use in fixed-bed reactors, for example. Theruggedness of the structures, the porosity (both pore volume and porestructure), and the purity of the carbide nanorods and/or oxycarbidenanorods are significantly improved. Combining these properties withrelatively high surface areas provides a unique material with usefulcharacteristics.

One embodiment of the invention relates to a rigid porous structurecomprising carbide nanorods having an accessible surface area greaterthan about 10 m²/gm and preferably greater than 50 m²/gm, beingsubstantially free of micropores and having a crush strength greaterthan about 1 lb. The structure preferably has a density greater than 0.5gm/cm³ and a porosity greater than 0.8 cm³/gm. Preferably, the structurecomprises intertwined, interconnected carbide nanorods and issubstantially free of micropores.

According to one embodiment, the rigid porous structure includes carbidenanorods comprising oxycarbide compounds, has an accessible surface areagreater than about 10 m²/gm, and preferably greater than 50 m²/gm, issubstantially free of micropores, has a crush strength greater thanabout 1 lb and a density greater than 0.5 gm/cm³ and a porosity greaterthan 0.8 cm³/gm.

According to another embodiment the rigid porous structure includesoxycarbide nanorods having an accessible surface area greater than about10 m²/gm, and preferably greater than 50 m²/gm, being substantially freeof micropores, having a crush strength greater than about 1 lb, adensity greater than 0.5 gm/cm³ and a porosity greater than 0.8 cm³/gm.

According to yet another embodiment, the rigid porous structure includescarbon nanotubes comprising a carbide portion. The location of thecarbide portion can be on the surface of the carbon nanotube or anyplace on, in or within the carbon nanotube or the carbide portion can beconverted into a carbide nanorod forming a carbon nanotube-carbidenanorod hybrid. Nevertheless, the catalytic effectiveness of these rigidporous structures is not affected by the carbide portion on theresulting composites. This rigid porous structures has an accessiblesurface area greater than about 10 m²/gm and preferably than 50 m²/gm,is substantially free of micropores, has a crush strength greater thanabout 1 lb, a density greater than 0.5 gm/cm³ and a porosity greaterthan 0.8 cm³/gm.

In another related embodiment the rigid porous structure includes carbonnanotubes having a carbide portion and also an oxycarbide portion. Thelocation of the oxycarbide portion can be on the surface of the carbideportion or any place on, in or within the carbide portion.

Under certain conditions of oxidative treatment it is possible toconvert a portion of the carbide nanorod part of the carbon-carbidenanotube-nanorod hybrid into an oxycarbide. The rigid porous structureincorporating carbon-carbide-oxycarbide nanotube-nanorod hybrids has anaccessible surface area greater than about 10 m²/gm, is substantiallyfree of micropores, has a crush strength greater than about 1 lb, adensity greater than 0.5 gm/cm³ and a porosity greater than 0.8 cm³/gm.

According to one embodiment, the rigid porous structures described abovecomprise nanotubes and/or nanorods which are uniformly and evenlydistributed throughout the rigid structures. That is, each structure isa rigid and uniform assemblage of nanotubes and/or nanorods. Thestructures comprise substantially uniform pathways and spacings betweenthe nanotubes and/or nanorods. The pathways or spacings are uniform inthat each has substantially the same cross-section and are substantiallyevenly spaced. Preferably, the average distance between nanotubes and/ornanorods is less than about 0.03 μm and greater than about 0.005 μm. Theaverage distance may vary depending on the density of the structure.

According to another embodiment, the rigid porous structures describedabove comprise nanotubes and/or nanorods which are nonuniformly andunevenly distributed throughout the rigid structures. The rigidstructures comprise substantially nonuniform pathways and spacingsbetween the nanorods. The pathways and spacings have nonuniformcross-section and are substantially unevenly spaced. The averagedistance between nanotubes and/or nanorods varies between 0.0005 μm to0.03 μm. The average distances between nanotubes and/or nanorods mayvary depending on the density of the structure.

According to another embodiment, the rigid porous structure comprisesnanotubes and/or nanorods in the form of nanotube and/or nanorodaggregate particles interconnected to form the rigid structures. Theserigid structures comprise larger aggregate spacings between theinterconnected aggregate particles and smaller nanotube and/or nanorodspacings between the individual nanotubes and/or nanorods within theaggregate particles. Preferably, the average largest distance betweenthe individual aggregates is less than about 0.1 μm and greater thanabout 0.001 μm. The aggregate particles may include, for example,particles of randomly entangled balls of nanotubes and/or nanorodsresembling bird nests and/or bundles of nanotubes and/or nanorods whosecentral axes are generally aligned parallel to each other.

Another aspect of the invention relates to the ability to provide rigidporous particulates or pellets of a specified size dimension. Forexample, porous particulates or pellets of a size suitable for use in afluidized packed bed. The method involves preparing a plurality ofnanotubes and/or nanorods aggregates, fusing or gluing the aggregates ornanotubes and/or nanorods at their intersections to form a large rigidbulk solid mass and sizing the solid mass down into pieces of rigidporous high surface area particulates having a size suitable for thedesired use, for example, to a particle size suitable for forming apacked bed.

General Methods of Making Rigid Porous Structures

The above-described rigid porous structures are formed by causing thenanotubes and/or nanorods to form bonds or become glued with othernanofibers at the fiber intersections. The bonding can be induced bychemical modification of the surface of the nanofibers to promotebonding, by adding “gluing” agents and/or by pyrolyzing the nanofibersto cause fusion or bonding at the interconnect points. U.S. Pat. No.6,099,965 to Tennent describes processes for forming rigid porousstructures from carbon nanotubes. These processes are equally applicableto forming rigid porous structures including discrete unstructurednanotubes or nanotube aggregates comprising carbides and in anotherembodiment also oxycarbides, wherein the carbon nanotube morphology hasbeen substantially preserved. These methods are also applicable toforming rigid porous structures comprising carbide or oxycarbidenanorods, unstructured or as aggregates. Additionally, these methods arealso applicable to forming rigid porous structures comprising hybrids ofcarbon-carbide nanotube-nanorods and/or carbon-carbide-oxycarbidenanotube-nanorods.

In several other embodiments rigid porous structures comprising carbidenanorods are prepared by contacting a rigid porous carbon structure madeof carbon nanotubes with volatile Q-containing compounds underconditions sufficient to convert all of the carbon or only part of thecarbon of the carbon nanotubes to carbide-containing compounds.

Methods of Making Carbide Containing Rigid Porous Structures

There are many methods of preparing rigid porous structures comprisingcarbide nanorods. In one embodiment the rigid porous carbon structuresprepared as described above are contacted with Q-containing compoundsunder conditions of temperature and pressure sufficient to convert thecarbon nanotubes of the rigid porous carbon structure to carbidenanorods. The carbide portion of the carbon nanotubes of the rigidporous structure can be on the surface of the carbon nanotube or at anyplace on, in or within the carbon nanotube. When the conversion iscomplete, the entire carbon nanotube is transformed into a substantiallysolid carbide nanorod. Once in the possession of the teachings herein,one of ordinary skill in the art can determine as a routine matter andwithout the need for undue experimentation how to control the rate ofconversion of carbon nanotubes present in the rigid porous carbonstructure to a rigid porous carbide-containing structure comprisingcarbon nanotubes having a carbide portion located at various places onthe carbon nanotube present in an amount from about 20% to about 85%,preferably in excess of 85% by weight.

The carbide-containing rigid porous structures of the present inventionhave high accessible surface areas between 10 m²/gm and 100 m²/gm andare substantially free of micropores. These structures have increasedmechanical integrity and resistance to attrition in comparison toindividual carbide-containing nanorods. Carbide-containing rigid porousstructures have a density greater than 0.5 gm/cm³ and a porosity greaterthan 0.8 cm³/gm. The structure has at least two dimensions of at least10 μm and not greater than 2 cm. Depending on the pore structure of thestarting rigid porous carbon structure, the structure of thecarbide-containing rigid porous structure can be uniform, nonuniform orbimodal.

When the rigid porous structure is uniform the average distance betweenthe carbide-containing nanorods is less than 0.03 μm and greater than0.005 μm. In another embodiment the rigid porous structure comprisescarbide-containing nanorods in the form of interconnected aggregateparticles wherein the distance between individual aggregates ranges frompoint of contact to 1 μm. When the carbide-containing nanorod rigidporous structures are formed from rigid porous carbon structurescomprising nanotube aggregates, the structure has aggregate spacingsbetween interconnected aggregate particles and carbide nanorod spacingsbetween nanorods within the aggregate particles. As a result the rigidporous structure has a bimodal pore distribution.

One embodiment of the invention relates to rigid porous structurescomprising extrudates of aggregate particles of carbide nanorods,wherein the carbide nanorods are glued together with binding agents suchas cellulose, carbohydrates, polyethylene, polystyrene, nylon,polyurethane, polyester, polyamides, poly(dimethylsiloxane) and phenolicresins. Without being bound by theory, it is believed that theconversion of a rigid porous carbon structure to a carbide-containingrigid porous structure, whether completely or partially, is accomplishedin pseudotopotactic manner as previously discussed.

Methods of Making Oxycarbide Containing Rigid Porous Structures

There are many methods of preparing rigid porous structures comprisingoxycarbide nanorods and/or nanotubes comprising a carbide portion and anoxycarbide portion. In one embodiment the carbide containing rigidporous structures are subjected to oxidative treatments as disclosed inthe art and in U.S. Pat. No. 5,576,466.

In another embodiment rigid porous structures comprising carbonnanotubes having an oxycarbide portion and/or a carbide portion areprepared by subjecting rigid porous carbon structures which have beenpartially converted to carbide nanorods to oxidative treatmentsdisclosed in the art.

In another embodiment discrete carbide nanorods are subjected tooxidative treatments and then assembled into rigid porous structuresaccording to methods similar to those disclosed in U.S. Pat. No.6,099,965.

In yet another embodiment discrete carbon nanotubes or aggregate ofcarbon nanotubes which have been partially converted to carbide nanorodsare further subjected to oxidative treatments and then assembled intorigid porous structures according to methods disclosed in U.S. Pat. No.6,099,965.

Catalytic Compositions

The carbide and/or oxycarbide nanorods and nanotubes having carbideand/or oxycarbide portions of the invention, have superior specificsurface areas as compared to carbide and oxycarbide catalysts previouslytaught in the art. As a result, they are especially useful in thepreparation of catalysts and as catalyst supports in the preparation ofsupported catalysts. The catalysts of the invention include catalyticcompositions comprising nanotubes and/or nanorods and rigid porousstructures comprising the same. These “self-supported” catalysts of theinvention constitute the active catalyst compound and can be used withor without any additional physical support to catalyze numerousheterogeneous, fluid phase reactions as more specifically describedherein. The supported catalysts of the invention comprise a supportincluding a nanofiber and/or nanorod rigid porous structure and acatalytically effective amount of a catalyst supported thereon. Thecatalytic compositions can contain from about 10% to 95% carbides byweight of the composition. The catalyst compositions can further includefrom about 0.5% to 25% oxycarbides by weight of the carbides of thecomposition.

The uniquely high macroporosity of carbon nanotube or carbide nanorodstructures, the result of their macroscopic morphology, greatlyfacilitates the diffusion of reactants and products and the flow of heatinto and out of the self-supported catalysts. This unique porosityresults from a random entanglement or intertwining of nanotubes and/ornanorods that generates an unusually high internal void volumecomprising mainly macropores in a dynamic, rather than static state.Ease of separation of these catalysts from the fluid phase and lowerlosses of these catalyst as fines also improves process performance andeconomics. Other advantages of the nanotube and/or nanorod structures asself-supported catalysts include high purity, improved catalyst loadingcapacity and chemical resistance to acids and bases. As self-supportedcatalysts, carbon nanotube and/or nanorod aggregates have superiorchemical and physical properties in respect of their porosity, surfacearea, separability and purity.

Self-supported catalysts made of nanotubes and/or nanorods have a highinternal void volume that ameliorates the plugging problem encounteredin various processes. Moreover, the preponderance of large poresobviates the problems often encountered in diffusion or mass transferlimited reactions. The high porosities ensure significantly increasedcatalyst life.

One embodiment of the invention relates to a self-supported catalystwhich is a catalytic composition comprising carbide-containing nanorodshaving a diameter between at least 1 nm and less than 100 nm, andpreferably between 3.5 nm and twenty nm. The carbide-containing nanorodshave been prepared from carbon nanotubes which have been substantiallyconverted to carbide nanorods. In the catalytic compositions of thisembodiment the carbide nanorods retain substantially the structure ofthe original carbon nanotubes. Thus, the carbide nanotubes can haveuniform, nonuniform or bimodal porous structures. These catalyticcompositions can be used as catalysts to catalyze reactions such ashydrogenation, protonation, oxidation, hydrodesulfurisation,hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation,hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization,alkylation, dealkylation and transalkylation.

Catalytic Compositions Supported on Aggregates of Carbide and OxycarbideNanorods

Depending upon the application, the rigid porous structures of theinvention can be used as both self-supported catalysts and as catalystsupports. As is true of catalysts comprising regular nanotubes and/ornanorods, catalysts and catalyst supports comprising the rigid porousstructures of the invention have unique properties. They are relativelyfree of micropores. They are also pure and resistant to attrition,compression and shear. Consequently, they can be easily separated from afluid phase reaction medium and after a long service life. The rigidporous structures of the invention can be used as catalysts and catalystsupports in a variety of fixed bed catalytic reactions.

Rigid structures formed from nanorod aggregates, preferably siliconcarbide and aluminum carbide-containing nanorods, are particularlypreferred structures for use as catalyst supports.

The combination of properties offered by nanorod structures is unique.Known catalyst supports do not have such high porosity, high accessiblesurface area and attrition resistance. This combination of properties isadvantageous in any catalyst system amenable to the use of a carbidecatalyst support. The multiple nanorods that make up a nanorod structureprovide a large number of junction points at which catalyst particlescan bond to the structures. This provides a catalyst support thattenaciously holds the supported catalyst. Further, nanorod structurespermit high catalyst loadings per unit weight of nanorod. Catalystloadings are generally greater than 0.01 weight percent and preferablygreater than 0.1, but generally less than 5% weight containing on thetotal weight of the supported catalyst. Typically catalyst loadingsgreater than 5% by weight are not useful, but such catalyst loadings areeasily within the contemplation of the invention, as are loadings inexcess of 50 weight percent containing of the total weight of thesupported catalyst.

Desirable hydrogenation catalysts which can be supported on the nanorodand/or nanotube structures of the invention are the platinum group ofmetals (ruthenium, osmium, rhodium, iridium, palladium and platinum or amixture thereof), preferably palladium and platinum or a mixturethereof. Group VII metals including particularly iron, nickel and cobaltare also attractive hydrogenation catalysts.

Oxidation (including partial oxidation) catalysts may also be supportedon the nanotube and/or nanorod structures. Desirable metallic oxidationcatalysts include, not only members of the platinum group enumeratedabove, but also, silver and the group VIII metals. Oxidation catalystsalso include metal salts known to the art including salts of vanadium,tellurium, manganese, chromium, copper, molybdenum and mixtures thereofas more specifically described in Heterogeneous Catalytic ReactionsInvolving Molecular Oxygen, by Golodets, G. I. & Ross, J. R. H, Studiesin Surface Science, 15, Elsevier Press, NYC 1983.

Active catalysts include other carbide compounds such as carbides oftitanium, tantalum, hafnium, niobium, zirconium, molybdenum, vanadiumand tungsten. These carbides are particularly useful for hydrogenation,protonation, oxidation, hydrodesulfurisation, hydrodenitrogenation,hydrodemetallisation, hydrodeoxygenation, hydrodearomatization,dehydrogenation, hydrogenolysis, isomerization, alkylation, dealkylationand transalkylation.

Because of their high purity, carbide nanorod aggregates exhibit highresistance to attack by acids and bases. This characteristic isadvantageous since one path to regenerating catalysts is regenerationwith an acid or a base. Regeneration processes can be used which employstrong acids or strong bases. This chemical resistance also allows thecarbide supports of the invention to be used in very corrosiveenvironments.

Preparation of Supported Catalysts

Supported catalysts are made by depositing a catalytically effectiveamount of catalyst on the rigid nanorod and/or nanotube structure. Theterm “on the nanotube and/or nanorod structure” embraces, withoutlimitation, on, in and within the structure and on the nanotubes and/ornanorods thereof. These terms may be used interchangeably. The catalystcan be incorporated onto the nanotube and/or nanorod or aggregatesbefore the rigid structure is formed, while the rigid structure isforming (i.e., it can be added to the dispersing medium) or after therigid structure is formed.

Methods of depositing the catalyst on the support include adsorption,incipient wetness, impregnation and precipitation. Supported catalystsmay be prepared by either incorporating the catalyst onto the aggregatesupport or by forming it in situ and the catalyst may be either activebefore it is deposited in the aggregate or it may be activated in situ.

Catalysts such as a coordination complexes of catalytic transitionmetals, e.g., palladium, rhodium or platinum, and a ligand, such as aphosphine, can be adsorbed on a support by slurrying nanorods in asolution of the catalyst or catalyst precursor for an appropriate timeto achieve the desired loading.

These and other methods may be used in forming the catalyst supports. Amore detailed description of suitable methods for making catalystsupports using nanotube structures is set forth in U.S. Pat. No.6,099,965.

Catalytic Compositions and Their Uses

The above described catalytic compositions are suited for use in fluidphase reactions such as hydrogenation, protonation, oxidation,hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation,hydrodeoxygenation, hydrodearomatization, dehydrogenation,hydrogenolysis, isomerization, alkylation, dealkylation, andtransalkylation.

Another embodiment of the invention relates to a catalyst comprising acomposition including a multiplicity of Q-based nanorods, wherein Q isselected from the group consisting of B, Si, Al, Ti, Ta, Nb, Zr, Hf, Mo,V and W. The resulting carbide nanorods can be distributed nonuniformly,uniformly or can be in the form of interconnected aggregate particles.

In a related embodiment, the catalyst comprises a rigid porous structurebased on the Q-based nanorods described above which have been formedinto extrudates and connected by gluing agents or in any other mannersufficient to form the rigid porous structure. Each catalyticcomposition discussed immediately above can be used as catalysts in afluid phase reaction selected from the group consisting ofhydrogenation, protonation, oxidation, hydrodesulfurisation,hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation,hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization,alkylation, dealkylation and transalkylation.

Another embodiment relates to a catalyst comprising a compositionincluding a multiplicity of carbide-based nanorods which furthercomprise oxycarbide compounds any place on, in or within the nanorod,preferably on the surface.

In a related embodiment the catalyst comprises a rigid porous structureincluding the carbide-based nanorods comprising oxycarbides which havebeen formed into extrudates connected into the rigid porous structure bygluing agents or in any other manner sufficient to form the rigid porousstructure. Each catalytic compositions discussed immediately above canbe used as a catalyst in a fluid phase reaction selected from the groupconsisting of hydrogenation, protonation, oxidation,hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation,hydrodeoxygenation, hydrodearomatization, dehydrogenation,hydrogenolysis, isomerization, alkylation, dealkylation andtransalkylation.

Another embodiment relates to a catalyst comprising a compositionincluding a multiplicity of carbon nanotubes having substantiallyuniform diameters. In this embodiment the carbon nanotubes comprisecarbide compounds anywhere on, in or within the nanotubes, butpreferably on the surface of the nanotubes. In yet another relatedembodiment the carbon nanotubes additionally comprise oxycarbidecompounds on, in or within the nanotubes, but preferably on the surfaceas more specifically described in section “Supported Carbides andOxycarbides” of the of the specification. In these embodiments thenanotube morphology is substantially retained.

In a related embodiment the catalyst comprises a rigid porous structureincluding carbon nanotubes comprising carbide compounds and, in anotherembodiment, also oxycarbide compounds as described above. Each rigidporous structure is useful as a catalyst in a fluid phase reaction tocatalyze a reaction selected from the group consisting of hydrogenation,protonation, oxidation, hydrodesulfurisation, hydrodenitrogenation,hydrodemetallisation, hydrodeoxygenation, hydrodearomatization,dehydrogenation, hydrogenolysis, isomerization, alkylation, dealkylationand transalkylation.

In another embodiment the catalytic composition includes a multiplicityof carbon nanotubes having a carbide portion which has been converted toa carbide nanorod forming a nanotube-nanorod hybrid structure. Inanother related embodiment, the catalytic composition includes amultiplicity of carbon nanotubes having a carbide nanorod portion and inaddition also an oxycarbide portion which has been converted to anoxycarbide nanorod. In yet other related embodiments the foregoingcarbon nanotubes can be included in rigid porous structures, wherein thecarbon nanotubes are formed into extrudates and/or are otherwiseconnected to form rigid porous structures. The catalytic compositionsare useful as catalysts in a fluid phase reaction.

Fluid Phase Reactions Using Catalysts Containing Carbide or Oxycarbides

Carbide and/or oxycarbide catalysts can be used to catalyze fluid phasereactions such as hydrogenation, protonation, oxidation,hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation,hydrodeoxygenation, hydrodearomatization, dehydrogenation,hydrogenolysis, isomerization, alkylation, dealkylation andtransalkylation.

For example for isomerization reactions, such as the isomerization ofalkanes, any straight-chain, branched or cyclic alkane can be employedas a feed hydrocarbon in the isomerization process. Examples of alkaneswhich can be isomerized include, but are not limited to, n-butane,n-pentane, n-hexane, 2-methylpentane, 3-methylpentane, n-heptane,2-methylhexane, 3-methylhexane, octanes, nonanes, decanes andcombinations thereof. Alkenes, or olefins, can also be isomerized usingthe catalysts.

Any suitable isomerization conditions can be employed in the process ofthe invention. A feed hydrocarbon and a carrier gas such as hydrogen arepremixed to create an isomerization feed stream which is then charged toan isomerization zone, i.e., a vapor phase reactor vessel. The feedstream contacts a catalytic composition of the invention that has beenplaced within the reactor vessel.

The effluent from the reactor vessel is subjected to suitable separationtechniques as known in the art, to separate the desired isomer productfrom reactants and byproducts. For example, in the isomerization ofnormal butane to isobutane, the following process can be used. A vaporphase reactor is constructed by assembling a thermowell through an endof a vertical quartz tube. A vertical quartz tube with a 12 mm outerdiameter is fitted with a 6 mm quartz thermowell. At the tip of thethermowell, is a porous plug of quartz wool. This porous plug is used tosupport the catalyst and/or the catalyst support containing thecatalyst. The entire tube is then placed in a tube furnace, for examplea ½-inch tube furnace, in a vertical orientation. The top of the tube isfitted with inlet lines for the feed stream of reactant gases. At thebottom of the tube is an exit line connected to a pressure gauge, e.g.,a gauge measuring pressure between 0 and 15 psi. Mass flow controllersare placed in the gas inlet lines to control the flow of reactant gasesinto the reactor. Suitable mass flow controllers include thosemanufactured by Allborg Instruments and Controls of Orangeburg, N.Y.

Ground catalyst or supported catalyst is placed onto the quartz woolplug. The catalytic composition is then treated with hydrogen and argongas. Thereafter, the catalytic composition can be treated with oxygenand argon gas to make oxycarbide compositions.

Mixtures of the gas to be isomerized, e.g., n-butane, and hydrogen inmolar ratios of about 1:16 to 1:4 of n-butane:H₂ are introduced into thevapor phase reactor at WHSVs of 1–10 h⁻¹. Temperatures and pressures forthe reaction can range from 100° C. to about 400° C., and 1 to about 10psi, respectively.

Product gases are fed through a gas sampling valve. Gas chromatography(“GC”) can be used to analyze the composition of the product gas anddetermine the conversion and selectivity of the reaction. For example, aVarian gas chromatograph equipped with a GS-Q capillary column can beused to measure C₁–C₅ alkanes and olefins. The capillary columns can beobtained from Alltech Associates of Deerfield, Ill.

Hydrogenation reactions can also be catalyzed by the catalysts of thepresent invention. The catalysts can be used in any hydrogenationreactions that traditionally use noble metals such as platinum orrhenium; the carbide and/or oxycarbide catalysts replace the noble metalcatalyst. Hydrocarbons that can hydrogenated include, but are notlimited to, hydrocarbons with unsaturated carbon-carbon double or triplebonds, as well as unsaturated alcohols, aldehydes, ketones, esters, andany other hydrocarbon that has unsaturated double and/or triple bonds.The reaction conditions for the hydrogenation reaction vary with thetype of hydrocarbon being hydrogenated. One of ordinary skill in the artwould be aware of the proper reaction conditions.

Hydrodesulfurisation reactions can also be catalyzed by the carbideand/or oxycarbide catalysts. Sulfur containing compounds that canundergo hydrodesulfurisation include but are not limited to thiophene,dibenzylthiophene and dimethyl dibenzyl thiophene. For example, reactionconditions can include temperatures ranging from 250 to 400° C. andpressures ranging from 1 to 10 MPa.

EXAMPLES

The examples are illustrative and not to be considered restrictive ofthe scope of the invention. Numerous changes and modification can bemade with respect to the invention. The materials used in the examplesherein are readily commercially available.

In all of the experiments which follow, aggregates of carbon nanotubesas manufactured by Hyperion Catalysis International of Cambridge, Mass.were used. The aggregates of carbon nanotubes were of the cotton candy(“CC”) morphology also known as combed yarn (“CY”) as described in thesection entitled “Nanotube Aggregates and Assemblages”.

Example 1 Preparation of Molybdenum Carbide Precursors by Impregnationof Carbon Nanotube Aggregates with Molybdenum Acetyl Acetonate

Five gms of powder samples of CC aggregates having porosity of 6.5 cc/gmwere impregnated by the incipient wetness method with 35 cc of anethanol solution containing the correct amount of MoO₂(C₅ H₇ O₂)₂ or(molybdenum acetyl acetonate, referred to as Moacac) necessary for thedesired C:Mo atom ratio loading. The resulting mixture was dried at 110°C. at full vacuum for 18 hours during which the Mo precursor decomposedto a mixture of molybdenum suboxides, generally designated as MoO_(3-x),wherein x is 0 or 1. The sample was set aside for conversion to carbidecatalysts by careful calcination under an inert atmosphere as describedin Examples 5, 6 or 7 below.

Example 2 Preparation of Molybdenum Carbide Precursors by Impregnationof Carbon Nanotube Aggregates with Ammonium Molybdate

A similar procedure as used in Example 1 above was followed, except thatthe impregnating solutions were aqueous solutions containing the correctamount of ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇ O₂₄.4H₂O,referred to as ammonium molybdate) necessary for the desired C:Mo atomratio loading. The resulting mixtures were dried at 225° C. in fullvacuum for 18 hours during which the heptamolybdate compound wasdecomposed to MoO₃. The sample was set aside for conversion to carbidecatalysts by careful calcination under an inert atmosphere as moreparticularly described in Examples 5, 6 and 7 herein.

Example 3 Preparation of Molybdenum Carbide Extrudate Precursors byImpregnation with Molybdenum Acetyl Acetonate or Ammonium Molybdate

CC or CY type aggregates were oxidized with nitric acid as described inU.S. Pat. No. 6,203,814 to Fischer to form oxidized CC aggregates havingan acid titer of about 0.6 meq/gm.

Five gms of the oxidized CC type aggregates of carbon nanotubes werewell-mixed with either an ethanol solution of Moacac or an aqueoussolution of ammonium heptamolybdate tetrahydrate, each solutioncontaining the correct amount of Mo compound necessary for the desiredC:Mo loading. The mixing was accomplished by kneading in a Braybenderkneader until the paste had a homogeneous consistency. The excesssolvent was removed from the kneaded sample by evaporation until asolids content of from about 8 to about 10% by weight was obtained. Thematerial was then extruded by using a pneumatic gun extruder. Theextrudates were about ⅛″ in diameter and several centimeters in length.The extrudates were then dried at 200° C. in air for 18 hours duringwhich some shrinkage occurred. The dried extrudates were then brokeninto pieces of about 1/16″ by ¼″ which were set aside for conversion tocarbide catalysts by careful calcination as described in Examples 5, 6and 7 herein.

Example 4 Preparation of Molybdenum Carbide Precursor by Mixing CarbonNanotube Aggregates With Ammonium Molybdate or Molybdenum Oxide

As grown CC or CY aggregates were oxidized with nitric acid as describedin Example 3 to form oxidized CC aggregates having an acid titer ofabout 0.6 meq/gm.

Five gms of oxidized CC type aggregates of carbon nanotubes werephysically admixed with the correct amount of either ammoniumheptamolybdate tetrahydrate or MoO₃ necessary for the desired C:Mo atomratio by kneading the sample in a mortar and pestle. A small amount ofwetting agent such as water or ethylene glycol, was added periodicallyto keep the oxidized carbon nanotube powder dusting under control and tofacilitate the contact between the molybdenum precursor particles andthe carbon nanotube aggregates. After the mix was kneaded to ahomogeneous thick paste, the excess solvent was removed by gentlewarming while continuing to knead the sample. The mixture was then driedat 200° C. for 14 hours in air and set aside for conversion to carbideby careful calcination as described in Examples 5, 6 and 7 herein.

Example 5 Calcination of Molybdenum Carbide Precursors at 600° C. or625° C.

Weighed samples of molybdenum carbide precursors, as prepared inExamples 1–4 were loaded into porcelain boats which were then placedhorizontally in a one-inch quartz tube. The tube and boat assembly wereplaced in a high temperature furnace equipped with a programmabletemperature controller and a movable thermocouple. The thermocouple waslocated directly in contact with the end of the boat. The sample washeated under a slow flow, i.e., at several standard cc's/min of argon ata heating rate of 5° C./min to 200° C. and thereafter at 1° C./min tothe final temperature of 600° C. or 625° C. The sample was held at thistemperature for 18 hours. Since pure Mo₂C reacts violently withatmospheric oxygen, after cooling in argon to ambient temperature, thesamples were passivated by passing 3% O₂/Ar over them for 1 hour. XRDphase analysis indicated that the precursors have converted into β-Mo₂Cand/or γ-Mo₂C with minor component of MoO₂.

Example 6 Calcination of Molybdenum Carbide Carbon Precursors at 800° C.

The same procedure as described in Example 5 above was followed up to600° C. The samples were then held at 600° C. for 1 hour. Thereafter,heating was resumed at the same rate of 1° C./min to 800° C. and held atthat temperature for another 3 hours. After cooling in argon, thesamples were passivated using 3% O₂/Ar. XRD phase analysis indicatedthat the precursors have converted into β-Mo₂C.

Example 7 Calcination of Molybdenum Carbide Carbon Precursors at 1000°C.

The same procedure as described in Example 6 above was followed up to800° C., at which temperature the samples were held for 1 hour.Thereafter, heating of the samples was resumed at the rate of 1° C./minto 1000° C.•, where the temperature was maintained for 0.5 hours. Aftercooling in argon, the samples were passivated using 3% O₂/Ar. XRD phaseanalysis indicated that the precursors have converted into β-Mo₂C.

Results of Examples 1–7

Carbide nanorods and carbide nanoparticles supported on carbon nanotubeswere prepared according to Examples 1 to 7 above. Table 1 belowsummarizes the experimental conditions and XRD results for selectedexperiments.

TABLE 1 SUMMARY OF RESULTS FOR MOLYBDENUM CARBIDE PREPARATIONS C:MoWeight loss SAMPLE Mo Source T° C. initial (theor.) PHASES, XRD 1 Moacac(s)^(b) 600 4 (94)^(e) 27 (44) C, MoO₂, Mo₂C (hex) 2 MoO₃ (s)^(a) 800 35(38)^(e) 23 (35) C, Mo₂C (cub) 3 MoO₃ (s)^(a) 800 40 (34)^(e) n/a C,Mo₂C (hex) 4 Moacac (s)^(b) 800 10 (85)^(e) 31 (32) C, Mo₂C (hex) 5Moacac (s)^(b) 800 20 (57)^(e) 27 (22) Mo₂C (hex/cub), Mo 6 MoO₃ (s)^(b)1000 10 (85)^(e) 41 (32) Mo₂C (hex), Mo 7 MoO₃ (s)^(b) 1000 20 (57)^(e)27 (22) Mo₂C (hex/cub), Mo 8 MoO₃ (s)^(b) 1000 10 (85)^(e) 38 (32) C,Mo₂C (hex) 9 MoO₃ (s)^(b) 625 30 (43)^(e) 20 (17) C, Mo₂C (hex/cub) 10MoO₃ (s)^(b) 625 20 (57)^(e) 27 (22) C, Mo₂C (hex), MoO₂ 11 MoO₃ (s)^(b)1000 50 (28)^(e) 12 (11) C, Mo₂C (hex/cub) 12 MoO₃ (s)^(c) 800 3.5(100)^(e) 55 (55) Mo₂C (hex/cub) ^(a)Impregnated powder of aggregates ofcarbon nanotubes ^(b)Impregnated extrudates of aggregates of carbonnanotubes ^(c)Powder of aggregates of carbon nanotubes physically mixedwith Mo precursor ^(e)Calculated Mo₂C loading in final calcined productassuming full conversion of Mo precursor to Mo₂C

The chemical reaction involved in the preparation of Samples 1–12 is:2MoO₃+7C→Mo₂C+6 CO.

In the second column is a list of molybdenum precursors converted toMo₂C by reaction with carbon nanotubes. Moacac refers to molybdenylacetylacetonate, and MoO₃ refers to molybdenum trioxide. “(s)” refers tothe solid phase of the molybdenum precursor. Superscripts a, b and crefer to methods of dispersing the reactants as described in Examples 2,3 and 4, respectively. T° C. refers to the final calcination temperatureof the reaction temperature cycle. “C:Mo initial” refers to the atomicratio of C:Mo in the original reaction mixture before conversion to acarbide compound. For example, the stoichiometric atomic ratio toproduce pure carbide with no excess C or Mo, i.e., pure Mo₂C is 3.5. Thenumber following in parentheses is the calculated loading of the Mo₂Ccontained in the resulting materials. “Weight loss (theor.)” refers tothe theoretical weight loss according to the chemical equation. “Phases,XRD” shows the compounds found in the XRD analyses. Mo₂C exists in 2distinct crystallographic phases, hexagonal and cubic.

TABLE 2 SUMMARY OF XRD RESULTS Mo₂C Mo₂C Sample (hex) (cubic) MoO₂MoC >100 nm  1 15~20 nm minor component  2 5~8 nm  3 5~8 nm  4 10~15 nm 5 15~20 nm ~15 nm  6 20 nm  7 36~38 nm  8 8~10 nm 8~10 nm  9 18 nmminor component 10 20~25 nm 5~8 nm 11 35 nm 12 26 nm

Table 2 summarizes the XRD results for the experiments summarized inTable 1, identifies the compounds made, the phases present and thecalculated average particle size for the different phases.

The average particle size is a volume-biased average size, such that thevalue of one large particle counts more heavily than several mediumparticles and much more than the volume of many small particles. This isa conventional procedure which is well known to those familiar with XRDmethods.

Discussion of Results of Examples 1–7

A. Unsupported Mo₂C Nanoparticles and Nanorods

Samples 1 and 12 provided the clearest evidence of the formation offree-standing Mo₂C nanorods and nanoparticles. These were obtained byreacting stoichiometric or near stoichiometric mixtures of MoO₃ andcarbon nanotubes, either as powder or as extrudates. Productidentification and morphologies were obtained by SEM, HRTEM and XRD. InExample 1, with about 15% excess of carbon, the major product wasidentified by XRD as the hexagonal phase of Mo₂C. MoO₂ and graphiticcarbon were seen as minor components. SEM showed the presence of bothnanorods (approximately 10 to 15 nm in diameter) and nanoparticles(approximately 20 nm).

Samples 11 and 12 were obtained by reaction carbon nanotubes with eithera stoichiometric mixture of well-dispersed MoO₃ powder or withimpregnated ammonium molybdate. More evidence of the formation of Mo₂Cnanorods and nanoparticles was obtained in Sample 12, obtained byreacting a stoichiometric mixture of MoO₃ and powder of carbonnanotubes. XRD, SEM and HRTEM analyses showed formation of both Mo₂Cnanorods and nanoparticles. The SEM analyses showed a network ofnanorods with nanoparticles distributed within the network as shown inFIG. 1. Accurate dimensions of carbide nanorods have been obtained byHRTEM as shown in FIG. 2, which shows carbide nanorods having diameterssimilar to those of carbon nanotubes, namely, about 7 nm. The carbidenanoparticles particles ranged from about 7 nm to about 25 nm indiameter.

Sample 12, a stoichiometric mixture, was studied in more detail in orderto learn the course of the reaction. The reaction was tracked bythermogravimetric analysis (TGA) as shown in FIG. 4. FIG. 4 shows thatthe stoichiometric reaction has occurred in two distinct steps, namely,reduction of MoO₃ by carbon to MoO₂ at from about 450 to about 550° C.,followed by further reduction to Mo₂C at from about 675° C. to about725° C. SEM and XRD analyses taken after calcination at 600° C. showed acomplete redistribution of oxide precursor from the very large,supra-micron particles of MoO₃ initially present to about 20 to 50 nmparticles of MoO_(3-x), well-dispersed amongst individual fibrils. Thisredistribution probably occurred through vaporization. Furthercalcination to 800° C. converted the MoO_(3-x) (wherein x is 0 or 1)mixture to Mo₂C nanorods and nanoparticles, with further reduction inparticle size from about 7 to about 25 nm. Without being bound by anytheory, even though redistribution of MoO₃ probably takes place throughvaporization, both chemical transformations (MoO₃→MoO₂ and MoO₂→Mo₂C) byreduction by carbon are believed to occur through solid-solid phasereactions.

B. Mo₂C Nanoparticles Supported on Carbon Nanotubes

XRD, SEM and HRTEM analyses of products from Sample 10 provided evidencefor the successful preparation of nanoparticles of Mo₂C supported onindividual carbon nanotubes. These products were formed by impregnationof ammonium molybdate from aqueous solution onto CC aggregates of carbonnanotubes and carefully calcined as shown in Table 1. XRD's of bothproducts showed the cubic form of Mo₂C to be the major component alongwith graphitic carbon. Hexagonal Mo₂C was seen as a minor component. Nomolybdenum oxide was detected. The cubic Mo₂C particles ranged fromabout 2 to about 5 nm in diameter, while the hexagonal particles rangedfrom about 10 to about 25 nm. The cubic particles were mainly depositedon individual carbon nanotubes, while the hexagonal particles weredistributed between carbon nanotubes. These can be seen in FIGS. 3 and4, which are copies of HRTEM micrographs taken from Sample 10. In thesepictures, the particle size can be estimated by direct comparison withthe fibril diameters, which range from 7 to 10 nm.

Example 8 Preparation of Tungsten Carbide Precursors by Impregnationwith Ammonium Tungstate

The procedure used in Example 2 above was followed, except that theimpregnating solution was an aqueous solution containing the correctamount of ammonium paratungstate hydrate ((NH₄)₁₀W₁₂O₄₁.5H₂O 72% W,referred to as ammonium tungstate) necessary for the desired C:W atomratio loading (C:W mole ratios of 3.5:1, 10:1 and 20:1). The resultingmixture was dried at 225° C. in full vacuum for 18 hours during whichthe paratungstate compound was decomposed to WO₃. The sample was setaside for conversion to carbide catalysts by careful calcination underan inert atmosphere as more particularly described in Example 10.

Example 9 Preparation of Tungsten Carbide Precursors by Impregnationwith Phosphotungstic Acid

The procedure used in Example 8 above was followed, except that theimpregnating solution was an aqueous solution containing the correctamount of phosphotungstic acid (H₃ PO₄.12 WO₃.xH₂O, 76.6% W referred toas PTA), necessary for the desired C:W atom ratio loading (C:W moleratios of 3.5:1, 10:1 and 20:1.) The resulting mixture was dried at 225°C. in full vacuum for 18 hours during which the PTA decomposed to WO₃.The sample was set aside for conversion to carbide catalysts by carefulcalcination under an inert atmosphere as more particularly described inExample 10.

Example 10 Calcination of Tungsten Carbide Precursors at 1000° C.

The procedure described in Example 7 above was followed to convertprecursors of tungsten carbides to tungsten carbides. After cooling inargon, the samples were passivated using 3% O₂/Ar. Table 3 belowsummarizes the experimental conditions and XRD results for selectedexperiments.

TABLE 3 SUMMARY OF RESULTS FOR TUNGSTEN CARBIDE PREPARATIONS SAMPLE WSource T° C. C:W INITIAL PHASES, XRD 1 PTA and CC^(a) 1000 3.5:1  WC andW₂C 2 PTA and CC 1000 10:1 WC and W₂C 3 PTA and CC 1000 20:1 WC and W₂C4 A. Tung and CC^(b) 1000 3.5:1  WC, W₂C and possibly W 5 A. Tung and CC1000 10:1 WC and W₂C 6 A. Tung and CC 1000 20:1 WC and W₂C^(a)Impregnated powder of CC aggregates of carbon nanotubes by incipientwetness with phosphotungstic acid ^(b)Impregnated powder of CCaggregates of carbon nanotubes by incipient wetness with ammoniumparatungstate hydrate

The chemical reactions involved in the preparation of Samples 1–6summarized in table 3 are:WO₃(s)+4C→WC+3 CO and 2WO₃(s)+7C→W₂C+6 CO.

In the second column of Table 3 is a list of tungsten precursors whichwere converted to W₂C/WC by reacting with carbon nanotubes. PTA refersto phosphotungstic acid and A. Tung refers to ammonium paratungstatehydrate. “(s)” refers to the solid phase of the tungsten precursor. C:Wrefers to the ratio of C atoms to W atoms in the original mix. Thestoichiometric atom ratio to produce pure WC with no excess of C or W is4.0. To produce pure W₂C, the atom ratio C:W is 3.5. The XRD columnlists the compounds observed in the XRD analyses.

Examples 11–13 Preparation of a Catalyst Support of Extrudates ofSilicon Carbide Nanorods

SiC nanorods were prepared from Hyperion aggregates of carbon nanotubesin accordance with Example 1 of U.S. application Ser. No. 08/414,369filed Mar. 31, 1995 by reacting the carbon nanotubes with ISO vapor athigh temperature. The resulting SiC nanorods have a uniform diameter offifteen nm on average and a highly crystallized β-SiC structure.

Poly(dimethylsiloxane) as provided by Aldrich Chemicals of Milwaukee,Wis. was used as a binder for the preparation of extrudates of SiCnanorods. 0.16 g of SiC nanorods and 0.16 g of poly(dimethylsiloxane)were mixed to form a uniform thick paste. Subsequently, the paste waspushed through a syringe to produce extrudates having a green colorwhich were heated under flowing argon atmosphere under the followingconditions: at 200° C. for 2 hours (Example 11); at 400° C. for 4 hours(Example 12); and at 700° C. for 4 hours (Example 13). A rigid porousstructure of SiC nanorods was formed.

The extrudates obtained in Examples 11–13 had a density of 0.97 gm/ccand a bimodal pore structure. The macropores were 1 to 5 μm, as shown inFIG. 5B among aggregates and the mesopores were 10 to 50 nm, as shown inFIG. 5C in the networks of intertwined SiC nanorods. The diameter of theextrudates was about 1.2 nm as shown in FIG. 5A. The specific surfacearea of the extrudates of SiC nanorods was 97 m²/gm.

Because of their high surface area, unique pore structure and hightemperature stability, the SiC extrudates are attractive for variousapplications, including as supports for catalysts such as platinum,palladium other catalytic metals and carbides of Mo, W, V, Nb or Ta. Thesurface properties of SiC nanorods when used as a catalyst support arevery close to those of carbon. Conventional carbon supports cantherefore be replaced with SiC extrudates. Many properties of carbonsupported catalysts can be realized in high temperature conditions, asrequired, in particular, for oxidation reactions.

Examples 14 and 15 Preparation by Reductive Carburization of Extrudatesof Carbon Nanotubes Including Molybdenum Carbides

Two samples of 5 gms of extrudates of carbon nanotubes having a volatilemolybdenum compound on the surface thereof prepared according to Example14 are charged into alumina boats. Each boat is placed into a tubefurnace and heated under flowing argon for 2 hours at 250° C. and 450°C., respectively. The gas is changed from argon to a mixture of CH₄/H₂(20% CH₄) and the furnace is slowly (1° C./min) heated up to 650° C.where the temperature is maintained for 1 hour. Molybdenum carbidessupported on the surface of the extrudates of the carbon nanotubes areobtained.

Example 16 Preparation by Reactive Chemical Transport of Extrudate ofMolybdenum Carbide Nanorods

One gram of an extrudate of carbon nanotubes, 8 gms of molybdenum powderand 50 mg of bromine contained in a glass capsule are placed into aquartz tube which is evacuated at 10⁻³ Torr and then sealed. After thebromine capsule is broken, the tube is placed into a tube furnace andheated at 1000° C. for about 1 week. The extrudates of carbon nanotubesare substantially converted to molybdenum carbide nanorods.

Example 17 Preparation by Carburization of Molybdenum Carbides Supportedon the Surface of Extrudates of Carbon Nanotubes

A sample of an extrudate of carbon nanotubes is placed in a verticalreactor such that a bed is formed. The extrudate is heated under flowingH₂ gas at 150° C. for 2 hours. Thereafter, the extrudate is cooled to50° C. H₂ gas passed through a saturator containing Mo(CO)₆ at 50° C. ispassed over the cooled extrudates of carbon nanotubes. As a result,Mo(CO)₆ becomes adsorbed on the surface of the extrudate. Following theadsorption of Mo(CO)₆, the temperature of the sample is raised to 150°C. in an atmosphere of pure H₂. The temperature is maintained at 150° C.for 1 hour. The temperature of the sample is then increased at 650° C.and maintained at this temperature for 2 hours under flowing H₂ gas. Asample of the extrudate of carbon having molybdenum on its surfaces isobtained. This sample is kept at 650° C. for 1 hour. The gas is switchedfrom H₂ to a CH₄/H₂ mixture (20% CH₄). The molybdenum adsorbed on thesurfaces of the carbon nanotubes is converted to molybdenum carbide. Theamount of molybdenum carbide formed on the surface of the extrudate canbe controlled, by varying the duration of adsorption of the Mo(CO)₆ overthe cooled carbon nanotube extrudate.

Example 18 Use of Mo₂C Nanoparticles and/or Nanorods Supported onAggregates of Carbon Nanotubes in the Vapor Phase Hydrogenation ofEthylene

A vapor phase reactor was assembled by inserting a 6 mm quartzthermowell upward through the bottom end of a vertical quartz tubehaving an outer diameter of 12 mm. A porous plug of quartz wool wasplaced at the tip of the thermowell to support a bed of catalyst powder.One gram of the catalyst of Sample No. 2 in Table 1 was crushed andsieved to (+80−60) standard mesh and placed onto the quartz wool plug.The tube was placed vertically into a ½ inch tube furnace as describedin Example 6. The top of the tube was fitted with gas inlet lines tofeed reactant gases, and the bottom of the tube was fitted with an exitline connected to a 0–15 psi pressure gauge. Rotometers in the gas inletlines controlled the flows and thus the ratios of the individualreactant gases. Product gases were fed through a gas sampling valve to aVarian gas chromatograph equipped with a GS-Q capillary column suitablefor analyzing C₁–C₅ alkanes and olefins.

Ethylene and hydrogen gases were fed to the vapor phase reactordescribed in molar ratios ranging from 1:1 to 4:1 ethylene:H₂ at 70° C.initial temperature, 1–3 psi gauge total pressure at a gas spacevelocity (GSV) 60–325 min⁻¹. In each run, the temperature of thecatalyst bed was held at the set temperature of 70° C. for severalminutes, and then rapidly increased to approximately 200° C. in severalminutes. The temperature of the catalyst bed remained relativelyconstant thereafter. Analyses by gas chromatography showed 100%conversion of ethylene to ethane. Ethane was the only product observed,i.e., the selectivities of the reaction approached 100%.

Example 19 Preparation of Oxycarbide Containing Nanorods Catalyst InSitu and Use of in the Vapor Phase Isomerization of Butane to Isobutane

The catalyst Sample 12 of Table 1 was oxidized to form oxycarbidenanorods. This unsupported catalyst was then used to isomerize butane toisobutane. A 1.0 gram sample of ground catalyst (+80−60) was placed in areactor as described in Example 18. The catalyst was treated with 10% H₂in argon at 700° C. for 30 minutes, cooled in argon to room temperature,and then treated with 3% O₂ in argon at 350° C. for 14 hours to obtainoxycarbide-containing nanorods. After purging the system of O₂, mixturesof n-butane and H₂ in molar ratios of n-butane:H₂ ranging from 1:16 to1:4 at 1–3 psi gauge pressure were fed to the reactor at a WHSV 1 hr⁻¹to 10 hr⁻¹. The products were analyzed by GC using a GS-Q capillarycolumn.

A steady yield of 4.4% isobutane was obtained at 380° C. at a WHSV 2hr⁻¹. Conversion of n-butane was approximately 4.5% with selectivity toisobutane of about 96% based on GC analyses. The byproducts, in order ofabundance, were propane, ethane and methane. Increasing the temperatureto 420° C. increased the conversion of n-butane to more than 10%isobutane. However, the selectivity to isobutane was less than 50%, withthe major selectivity loss to methane.

Example 20 Use of Mo₂C Nanoparticles and/or Nanorods Supported onExtrudates of Aggregates of Carbon Nanotubes in the Vapor PhaseHydrogenation of Ethylene

A vapor phase reactor is assembled as described in Example 18. One gramof the catalyst Sample No. 9 in Table 1 is crushed and sieved to(+80−60) standard mesh and placed onto the quartz wool plug. The tube isplaced vertically into a ½ inch tube furnace as described in Example 5.The top of the tube is fitted with gas inlet lines to feed reactantgases and the bottom of the tube is fitted with an exit line connectedto a 0–15 psi pressure gauge. Rotometers in the gas inlet lines controlthe flows and thus the ratios of the individual reactant gases. Productgases are fed through a gas sampling valve to a Varian gas chromatographequipped with a GS-Q capillary column.

Ethylene and hydrogen gases are fed to the vapor phase reactor in molarratios ranging from 1:1 to 4:1 ethylene: H₂ at 70° C. initialtemperature, 1–3 psi gauge total pressure at a GSV 60–325 min⁻¹. In eachrun, the temperature of the catalyst bed is held at the set temperatureof 70° C. for several minutes, then rapidly increased to approximately200° C. in several minutes. The temperature of the catalyst bed remainsrelatively constant thereafter. Analyses by GC shows 100% conversion ofethylene to ethane. Ethane is the only product observed, i.e.,selectivities approaching 100%.

Example 21 Use of Mo₂C Nanoparticles and/or Nanorods Supported onExtrudates of Aggregates of Carbon Nanotubes in the Vapor PhaseIsomerization of Butane to Isobutane

The catalyst of Sample 11 of Table 1 was oxidized to form oxycarbidenanorods which were used to isomerize butane to isobutane. A 1.0 gramsample of ground catalyst (+80−60) was placed in a reactor as describedin Example 18. In the reactor, the catalyst was then treated with 10% H₂in argon at 700° C. for 30 minutes, cooled in argon to room temperature,and then treated with 3% O₂ in argon at 350° C. for 14 hours to obtainoxycarbide nanorods. After purging the system of O₂, mixtures ofn-butane and H₂ in molar ratios of n-butane: H₂ ranging from 1:16 to 1:4at 1–3 psi gauge pressure were fed to the reactor at WHSVs ranging from1 to 10 hr⁻¹. The products were analyzed by GC using a GS-Q capillarycolumn provided by J&W of Alltech Associates.

A steady yield of 4.4% isobutane was obtained at 380° C. at a WHSV 2hr⁻¹. Conversion of n-butane was approximately 4.5% with selectivity toisobutane of about 96% based on GC analyses. The byproducts, in order ofabundance, were propane, ethane and methane. Increasing temperature to420° C. increased conversion of n-butane to more than 10%. Theselectivity to isobutane was less than 50%, with the major selectivityloss to methane.

Example 22 Use of Mo₂C Nanoparticles and/or Nanorods Supported onAggregates of Carbon Nanotubes in the Hydrodesulfurization of Thiophene

0.1 gm of the catalyst of Sample 2 in Table 1 is charged into a 500 ccstirred autoclave with 300 cc of 1 vol % solution of thiophene inhexadecane. The reactor is charged to 80 atm with H₂ and thehydrodesulfurization reaction is carried out at 300° C. One cc samplesare withdrawn and analyzed at 5-minute intervals and a pseudo firstorder rate constant for disappearance of thiophene is determined to be4.5×10⁻³ L/gm cat-s.

Example 23

Use of Unsupported Oxycarbide Nanorods as Catalyst in theHydrodesulfurization of Thiophene

0.1 gm of catalyst described in Example 19 above is charged into a 500cc stirred autoclave with 300 cc of 1 vol % solution of thiophene inhexadecane. The reactor is charged to 80 atm with H₂ and thehydrodesulfurization reaction is carried out at 300° C. One cc samplesare withdrawn and analyzed at 5 minute intervals and a pseudo firstorder rate constant for disappearance of thiophene is determined to be4.5×10⁻³ L/gm cat-s.

Example 24 Use of WC/W₂C Nanoparticles and/or Nanorods Supported onAggregates of Carbon Nanotubes in the Vapor Phase Hydrogenation ofEthylene

A vapor phase reactor is assembled as described in Example 18. One gramof the catalyst of Sample 1 in Table 3 is crushed and sieved to (+80−60)standard mesh and placed onto the quartz wool plug. The tube is placedvertically into a ½ inch tube furnace as described in Example 10. Thetop of the tube is fitted with gas inlet lines to feed reactant gases,and the bottom of the tube is fitted with an exit line connected to a0–15 psi pressure gauge. Rotometers in the gas inlet lines control theflows and thus the ratios of the individual reactant gases. Productgases are fed through a gas sampling valve to a Varian gas chromatographequipped with a GS-Q capillary column suitable for analyzing C₁–C₅alkanes and olefins.

Ethylene and hydrogen gases are fed to the vapor phase reactor in molarratios ranging from 1:1 to 4:1 ethylene: H₂ at 70° C. initialtemperature, 1–3 psi gauge total pressure and at GSVs of 65–325 min⁻¹.In each run, the temperature of the catalyst bed is held at the settemperature of 70° C. for several minutes, then it is rapidly increasedto approximately 200° C. in several minutes. The temperature of thecatalyst bed remains relatively constant thereafter. Analyses by GCshows 100% conversion of ethylene to ethane. Ethane is the only productobserved, i.e., the selectivities approach 100%.

Example 25 Preparation of Tungsten Oxycarbide Nanorods Catalyst in Situand Use of Same in the Vapor Phase Isomerization of Butane to Isobutane

The catalyst of Sample 2 of Table 3 is oxidized to form unsupportedoxycarbide nanorods which are used to isomerize butane to isobutane. A1-gram sample of ground catalyst (+80−60) is placed in a reactor asdescribed in Example 24. In the reactor, the catalyst is treated with10% H₂ in argon at 700° C. for 30 minutes, cooled in argon to roomtemperature, and then treated with 3% O₂ in argon at 350° C. for 14hours to obtain tungsten oxycarbide containing nanorods. After purgingthe system of O₂, mixtures of n-butane and H₂ in molar ratios ofn-butane:H₂ ranging from 1:16 to 1:4 at 1–3 psi gauge pressure are fedto the reactor at WHSVs of 1 to 10⁻¹. The products are analyzed by GCusing a GS-Q capillary column as provided by Alltech Associates.

A steady yield of 4.4% isobutane is obtained at 380° C. and at a WHSV of2 hr⁻¹. Conversion of n-butane is approximately 4.5% with selectivity toisobutane of about 96% based on GC analyses. The byproducts, in order ofabundance, are propane, ethane and methane. Increasing temperature to420° C. increases conversion of n-butane to more than 10% isobutane. Theselectivity to isobutane is less than 50% with the major selectivityloss to methane.

Example 26 Use of WC/W₂C Nanoparticles and/or Nanorods Supported onAggregates of Carbon Nanotubes in the Hydrodesulfurization of Thiophene

0.1 gram of the catalyst of Sample 1 in Table 3 is charged into a 500 ccstirred autoclave with 300 cc of 1 vol % solution of thiophene inhexadecane. The reactor is charged to 80 atm with H₂ and thehydrodesulfurization reaction is carried out at 300° C. One cc samplesare withdrawn and analyzed at 5-minute intervals and a pseudo firstorder rate constant for disappearance of thiophene is determined to be4.5×10⁻³ L/gm cat-s.

Example 27 Use of Unsupported Tungsten Oxycarbide Nanorods as Catalystin the Hydrodesulfurization of Thiophene

0.1 gram of the catalyst in Example 25 is charged into a 500 cc stirredautoclave with 300 cc of 1 vol % solution of thiophene in hexadecane.The reactor is charged to 80 atm with H₂, and the hydrodesulfurizationreaction is carried out at 300° C. One cc samples are withdrawn andanalyzed at 5-minute intervals and a pseudo first order rate constantfor disappearance of thiophene is determined to be 4.5×10⁻³ L/gm cat-s.

Example 28 Preparation of a Pd Catalyst Supported on SiC Extrudates.

A 5 weight % Pd/SiC extrudate catalyst is prepared by contacting 10.0 gmSiC extrudates prepared in Example 12 with a solution containing 1.455gms Pd(acetylacetonate)₂ (34.7% Pd, obtained from Alfa/Aesar of WardHill, Mass.) dissolved in 500 cc toluene. The mixture is stirred lightlyfor 1 hour, after which the toluene is removed at reduced pressure and40° C. in a rotary evaporator. The resulting dark brown solids are driedat 80° C. overnight, then calcined at 350° C. in air for 16 hours.

Example 29 Preparation of a Pt Catalyst Supported on SiC Extrudates.

A 1 weight % Pt/SiC extrudate catalyst is prepared by contacting 10.0gms SiC extrudates prepared in Example 12 with a 6 N HCl solutioncontaining 0.174 gram PtCl₄ (58% Pt, obtained from Alfa/Aesar). Themixture is stirred lightly for 1 hour after which solvent is removed atreduced pressure and 70° C. in a rotary evaporator. The resulting brownsolids are dried at 140° C.

Example 30 Oxidation of CH₄ with the Pd/SiC Extrudate Catalyst Preparedin Example 28.

Five gms of catalyst prepared in Example 28 are packed into a vertical½″ stainless steel tubular reactor to a height of about 2½″. A wad ofquartz wool placed atop a ¼″ stainless steel thermowell inserted upwardfrom the bottom of the tube supports the catalyst bed. The inlet andoutlet of the tube reactor are fitted to allow passage of gas into thereactor, through the catalyst bed and out of the reactor. The effluentgas is analyzed by a gas chromatograph using a Poropak Q columnmanufactured by Millipore Corp. of Bedford, Mass., which allowsquantitative analysis of CH₄, CO and CO₂. Temperature of the reactor ismeasured by a thermocouple inserted in the thermowell. The reactor isthen placed in a 1″ Lindberg furnace to provide heat.

The catalyst is reduced in situ with 5% H₂/N₂ at 350° C. for 12 hours.After purging residual H₂ with N₂, at atmospheric pressure, a gasmixture comprising 4% O₂/1% CH₄/95% N₂ is passed over the catalyst bedat 450° C. at a total gas rate of 50 l (stp)/hr. Gas chromatographanalysis shows a conversion of CH₄>99 mole % at about 100% selectivityto CO₂.

Example 31 Oxidation of CO with the Pt/SiC Extrudate Catalyst Preparedin Example 29.

The reactor used in Example 30 is loaded with 5 gms of catalyst. Thecatalyst is reduced in situ with 5% H₂/N₂ for 2 hours at 350° C., afterwhich the reactor is purged of H₂ by N₂. At atmospheric pressure, a gasmixture comprising 5% O₂/1% CO and 94% argon is passed over the catalystat 300° C. at a total gas rate of 40 l (stp)/hr. Gas chromatographicanalyses show complete conversion of CO to CO₂.

The terms and expressions which have been employed are used as terms ofdescription and not of limitations, and there is no intention in the useof such terms or expressions of excluding any equivalents of thefeatures shown and described as portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention.

Thus, while there had been described what are presently believed to bethe preferred embodiments of the present invention, those skilled in theart will appreciate that other and further modifications can be madewithout departing from the true scope of the invention, and it isintended to include all such modifications and changes as come withinthe scope of the claims.

1. A method of hydrogenating ethylene comprising the steps of: preparingmolybdenum carbide nanorods supported on aggregates of carbon nanotubes;introducing said nanorods onto a quartz wool plug in a reactor;introducing ethylene and hydrogen gases into the reactor at a 1:1 to 4:1molar ratio of ethylene:H₂ at 70° C., 1–3 psi gauge total pressure at agas space velocity of 60–325 min⁻¹; maintaining the quartz wool plugtemperature at 70° C. to permit the conversion of ethylene to ethane;and increasing the quartz wool plug temperature to 200° C. to completethe conversion of ethyene to ethane.